Tech support scammers have started targeting antivirus customers and have diversified their techniques, according to reports from antivirus vendors Avast and ESET.Cold-calling scams that target English-speaking computer users have been a common occurrence during the past two years. The scammers usually pose as tech support engineers who work for Microsoft or ISPs in an attempt to trick victims into buying questionable security or PC optimization software.However, it seems that these attacks are becoming increasingly more targeted, with callers beginning to impersonate employees from companies that users have already entrusted with their computers' protection."During the past week or so, we have received some complaints and it appears that some of our customers are being targeted by a new scam," said Adam Riley, head of third party suppliers at antivirus firm Avast Software, in a blog post on Tuesday.A small number of Avast customers have recently received unsolicited phone calls from people claiming to be Avast customer service representatives, who asked for money to fix problems identified on their computers, Riley said.However, Avast's customers are not the only antivirus users targeted by tech support scammers. "I recently learned from my colleagues at ESET UK that cold-callers from Mumbai have developed a new twist on this cold-calling scam, calling people in the UK and apparently claiming to offer paid support in response to problems that don't exist, because, they claim, 'ESET doesn't offer free support'," said David Harley, a senior research fellow at antivirus vendor ESET, in a blog post.Harley has been tracking tech support scams for a long time and has recently observed the development of new tactics by the people behind them.In order to trick victims into believing that their computers have a problem, scammers commonly leverage the Windows Event Viewer, a legitimate Microsoft application that lists various errors and warnings logged on the system. Most of them are not of critical importance, but can appear worrisome to non-technical users.However, attackers have recently started to ask users to open the Run dialog box by pressing Win key + R and type commands like "prefetch hidden virus" or "inf trojan malware," Harley said.The PREFETCH and INF commands open the C:WindowsPrefetch and C:WindowsInf folders respectively, which contain legitimate system files. However, less technical users might be inclined to think that these are malicious files revealed by the "hidden virus" or "trojan malware" command parameters."In fact, neither of these commands accepts parameters in the Run box," Harley said. "You could type 'inf elvish fantasy' or 'prefetch me a gin and tonic' and you'd get exactly the same directory listing, showing legitimate files." Fortunately, antivirus companies almost never contact their customers by phone regarding technical support issues, so receiving unsolicited phone calls from people claiming to represent such companies is a good indication of a scam, Harley said. "I can't guarantee that you won't get marketing calls but they should be within acceptable legal and ethical boundaries, and that doesn't include pretending to see malware on a system they don't have access to."Another sign that users are dealing with a scam is the caller's intention to use free or trial versions of commercial remote desktop software. "If a professional AV company needs access to your machine, they won't do it by misusing free licences for another company's software," Harley said.