A malicious program, dubbed QHosts, infects PCs using a recent flaw in Microsoft's Internet Explorer to take control of how computers look up Internet addresses, antivirus firms warned on Thursday.

The program takes advantage of a critical flaw in the popular Internet Explorer Web browser, which Microsoft has made an integral part of its Windows operating system. The flaw, which Microsoft has labeled an "object type" vulnerability, can be used to cause Web site visitors to unknowingly run malicious code onto their computers when surfing a compromised site. Such an attack is referred to as a Trojan horse.

The Trojan horse used a banner ad that the attacker somehow placed on Web hosting provider FortuneCity.com's site to infect PCs running Windows, said Craig Schmugar, a virus-research engineer with security company Network Associates. When a page containing the booby-trapped ad is displayed in Internet Explorer, the malicious code will automatically install the Trojan horse on the user's PC.

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