LONDON - Microsoft warned Thursday that hackers are actively exploiting a software vulnerability in PowerPoint, the company's presentation application.There's no patch yet for the bug, which could allow an attacker to completely control a computer. It affects Office 2000 Service Pack 3, Office XP SP3, Office 2003 SP3 and Office 2004 for Mac, Microsoft said in an advisory. Office 2007 is unaffected.Microsoft said it has seen limited, targeted attacks. The Danish security company Secunia ranked the problem as "extremely critical," its most severe rating.Secunia said the vulnerability "is caused due to an unspecified error that may result in access to an invalid object in memory when parsing a specially crafted PowerPoint file."Microsoft advised users to not open or save Office files that come from untrusted sources. If the file is opened, users won't have much of an indication that it's a malicious file."Usually, these files look legit when opened, so it is quite easy to fall prey and not even notice that something malicious ran in the background," according to a post on the company's Malware Protection Center blog.Hackers frequently hunt for vulnerabilities in commonly used software products, as it's another avenue other than the operating system to infect a computer with malicious software.To be hacked, a user would either have to download a malicious file hosted on a Web site or open a file sent through e-mail, Microsoft said.Microsoft has added a signature that will block access to PowerPoint exploit files in its Windows Live OneCare and Forefront Client security products. The signature is included in definition update 1.55.975.0 or higher.The company noted that the exploit files have been recently submitted to VirusTotal, a Web site that aggregates antivirus engines.Malware can be submitted to see which security products detect it. The site is often used by hackers to determine if their malware will be blocked by certain security software.