Depending on whom you ask, paying for antivirus software is either a good investment or a total ripoff. In reality, neither viewpoint is accurate. You can find plenty of good reasons to choose a paid antivirus product, and plenty of good reasons to go with a freebie.We teamed up with security testing company AV-Test, to find out what you get--or don't get--with free antivirus, and when it makes sense to subscribe annually to a fee-based program.Antivirus, DeconstructedFour basic levels of antivirus products exist: free, paid antivirus, suites, and "premium" suites. As you move up the ladder from free antivirus to premium suites, you typically get more features, such as identity theft protection, firewalls, parental controls, and system performance tools.Free antivirus software usually provides a bare minimum level of protection. It will scan for malware, and often can perform automatic scans, too. Some free apps may have additional protection tools such as a browser add-on that checks for bad links--and Comodo's free Internet Security Premium has a firewall. But such features are usually limited to paid antivirus products. Some free apps offer behavioral malware detection, which finds malware based on how it acts on your PC--a good way of detecting brand-new malware outbreaks. (Behavioral detection is standard on paid products.)Paid antivirus straddles a middle ground between the basic freebies and the feature-packed security suites: They typically offer more comprehensive security tools (such as parental controls and identity theft protection) and more flexibility than a free antivirus package, but they have fewer additional features than suites, which are intended to be one-stop security shops.One of the biggest drawbacks to going with a free product is the lack of technical support. While most companies offer some sort of phone support for paying customers, free antivirus users usually must fend for themselves. Avast does offer e-mail support for its free customers; most others provide only a knowledge base or forum where users can go for help.Another tradeoff is that free antivirus products often have some sort of advertisement for the company's paid product. Avast Free Antivirus has an upgrade link in the upper-right corner of the main window, and Avira AntiVir Personal will display an ad for Avira's paid antivirus software.How about malware signature updates? The security software companies I spoke to tell me that they treat their free and paid products the same as far as signature updates are concerned, although there may be some under-the-hood differences between their free and paid products (as is the case with Panda's software, for example). And one company, Avast, says that its free product is intended for average users, and that its paid antivirus is for more advanced users.On the other hand, free products do give you some flexibility. You can augment a free tool's basic security with countless security utilities. For instance, you can start with Avast Free Antivirus, add PCTools's Threatfire Free (which does a good job at bolstering malware detection), toss in one of the many firewalls available and a link-scanning utility to create your own custom security setup. This approach does require you to do your homework, though, and may be more complicated in the long run.InterfaceUser interfaces are typically as good (or as bad) in free products as they are in their paid counterparts. Avira and Avast, for example, use the same basic interface for their free and paid versions; they just include or leave out certain features and toggles as needed. Panda Cloud Antivirus (a freebie) is not a scaled-down version of the paid Panda Antivirus Pro, but rather a completely different product with a different interface and different internals.DetectionMost of the free products we tested put up identical or nearly identical malware detection scores to the paid varietals put out by the same company. But we did see some subtle differences. One notable example is Panda Cloud Antivirus: The free Cloud Antivirus and Panda's for-pay Antivirus Pro 2011 performed about the same on the signature-based malware detection tests, but Antivirus Pro did a better job in "real world" malware detection tests that help determine how well a product can block brand-new threats.(Note: Panda recently released version 1.3 of Panda Cloud Antivirus. The company says that version 1.3 should improve its detection of new malware, but the new release didn't come in time for us to test for our roundup. Check back here for future updates.)We found that, on the whole, paid antivirus products did a slightly better job at detecting malware than their freebie counterparts. In traditional signature-based detection tests, paid antivirus software that we tested found 96.2 percent of the malware samples overall. By comparison, free products' scores were ever-so-slightly worse, detecting 95.7 percent of samples.In real-world detection tests, free products missed 15.2 percent of samples, while paid products missed 10.2 percent of samples. When it came time to remove malware infections, again, the results were close, but paid antivirus software held a slight advantage.All the products we tested--both paid and free--detected all the test infections we threw their way, but paid products did a slightly better job overall at removing the active components of an infection, scoring a 74 percent success rate on average. The same held true when we tested how well the products removed all active and inactive components of an infection: Paid products achieved a 44 percent removal rate in this test, while free products averaged a full removal rate of 34 percentSpeedAlthough you lose some effectiveness against malware with free antivirus, you do gain a little speed. The free products we looked at were faster on average than the paid products in 9 of the 12 speed tests we take into account. Again, the difference was slight--the largest speed difference was around 10 percent. Both free and paid antivirus increased system startup times: A test PC with no antivirus software installed booted up in 40.1 seconds on average. With free antivirus installed, the test system started up in 44.1 seconds on average; and with paid antivirus installed, startup times increased to 46 seconds. It's hard to pin down the exact reason for this difference, but again, the difference is slight enough that it's basically statistically insignificant.Scan speeds, however, are a mixed bag. In our on-demand scan tests, paid antivirus programs edged out the freebies, scanning 4.5GB worth of files in an average of 2 minutes, 25 seconds. Free products, by comparison, completed the same test in an average of 2 minutes, 44 seconds.Free and paid products were nearly neck and neck in the on-access scan tests, which show how quickly the software can scan files for malware when they're opened or saved to disk. Using the same 4.5GB of files, free products completed the on-access scan test in an average time of 4 minutes, 50 seconds. Their paid counterparts completed the test 8 seconds later.Top PicksGiven how close the two classes of products are in terms of speed and effectiveness, the two biggest factors are features and customer support. With some exceptions, you get better customer support and more comprehensive security features with a paid product, but if you're willing to forgo these, it's definitely worth considering going free.Which free and paid antivirus products are best for you?Avast Free Antivirus comes out tops among free products thanks to its all-around strong malware-detection scores, good design, and low system drag. Avira AntiVir Personal takes second place: It put up solid malware-detection scores, but its interface is kludgy.Norton Antivirus leads the paid-software pack owing to its excellent malware detection, very good interface design, and comprehensive feature set that includes file-reputation analysis, a cloud-based scanner (which can help identify new malware more quickly), and system performance analysis tools. BitDefender Antivirus Pro 2011 is a close second thanks to strong malware detection; but slow scan speeds and some interface issues prevented it from taking first.Check out our lists of the top five free and top five paid antivirus products. Our summary reviews of the top two free and paid antivirus programs are below.Top Free Antivirus: Avast Avast Free Antivirus couples good all-around malware detection with a speedy, well-designed package. We liked its easy installation process, smooth interface design, and minimal impact on system performance.In traditional signature-based malware tests, Avast Free Antivirus detected 94.8 percent of samples, which is neither particularly good nor bad. It also did a decent, though not outstanding, job at detecting malware in our real-world malware detection tests: It completely blocked 76 percent of attacks (which is right about average), and partially blocked 4 percent of attacks.But on the plus side, Avast Free Antivirus didn't falsely identify a single "safe" file as a piece of malware, the only free product we looked at that achieved this. Avast Free Antivirus also did a good job at disinfecting a PC; it removed all active components of malware infections 80 percent of the time, which set the pace among the free products we looked at.Scan speeds are also very good. Its on-demand scan speed of 90 seconds was a close second to Avira AntiVir Personal, which completed the test in 87 seconds. And Avast Free Antivirus completed the on-access scan speed tests in 3 minutes, 40 seconds, tops among the products we tested.Top Paid Antivirus: Norton Antivirus 2011 Norton Antivirus 2011 ($40 for a one-year, single-PC license as of 10\/26\/2010) leads the pack of 2011 paid antivirus products. It does a very good job at detecting and removing malware, and has a smooth interface, although its speed is decidedly average.Installing Norton Antivirus 2011 is a breeze: The well-designed installer only requires you to click through two screens before it begins installing. Norton Antivirus has a smooth, polished interface: The main screen shows on-off toggles for its various protection features, as well as a map showing the global cybercrime activity over the last 24 hours.When it came to malware detection and removal, Norton Antivirus put out strong all-around scores. Norton Antivirus detected 98.7 percent of malware samples using traditional signature-based malware detection. This is an above average score, but it trails Panda Antivirus Pro 2011, which detected 99.8 percent of samples.Norton also put up excellent scores in the blocking of real-world malware attacks: Norton completely blocked 24 of the 25 samples we threw at it, but it did miss one sample completely. Norton also detected all active infections in our tests; it removed the active components of an infection 80 percent of the time, and was able to completely remove the infections 60 percent of the time--both above-average scores.Norton put up good scores in our speed testing, but not top-notch results. It completed an on-access scan of 4.5GB of files in 4 minutes, 32 seconds. This puts it ahead of the average of 4 minutes, 59 seconds, but it behind the top-performing product, which completed this test in 3 minutes, 35 seconds. Its on-demand scan speed of 2 minutes, one second is also above average, but again, it trails the top performing program by a good 30 seconds.