A new Trojan horse program was infecting PCs on Wednesday, exploiting a hole in Windows systems to sneak onto computers, then dropping adware or spyware or turning them into zombies, according to several Internet security companies.

The Trojan, dubbed Exploit-WMF (Windows Meta File), was rated a category 2 level risk, meaning it had the potential to continue to spread, said Dave Cole, director of security response at Symantec.

The exploit "is misusing a function in the WMF library in Windows," dropping onto the machine a downloader Trojan "that pulls down its big brother, a more sophisticated Trojan" from a server on the Internet, he said.

"Then it might try to pull down adware, spyware or a bot program," that can turn the computer into a zombie to be used for attacking other machines or sending spam, or just leave a hole on the computer through which sensitive data could be stolen, Cole said.

Kaspersky Lab rated the vulnerability "highly critical" and predicted that "new modifications of these programs may well appear in the near future."

The WMF vulnerability affects computers running Windows XP with service pack 1 and service pack 2, as well as Windows Server 2003 with service pack 0 and service pack 1. It can be exploited when an Internet Explorer user, or Firefox user under certain circumstances, visits a Web site that has malicious code on it or when a user previews .wmf format files with Windows Explorer, Kaspersky said in a statement.

The WMF library allows the computer to handle particular image types of Windows machines, Cole said. There is no patch for it yet from Microsoft, although antivirus vendors had released software to help protect against it, he said.

"Microsoft is investigating new public reports of a possible vulnerability in Windows and will continue to investigate the reports to help provide additional guidance for customers," a Microsoft spokesperson wrote in an e-mail. "Upon completion of this investigation, Microsoft will take the appropriate action to protect customers, which may include providing a fix through the monthly release process or issuing a security advisory, depending on customer needs."

Windows users can get more information about security issues at http://support.microsoft.com/security.