Some Internet Explorer users are also finding that malicious Web sites are secretly slipping trojan programs onto their computers, which could prove an even more dangerous exploit, said Drew Copley, a research engineer at Aliso Viejo, California-based eEye Digital Security, who discovered the original security vulnerability.

Such stealth programs can include keystroke loggers that record everything a person types or software to erase the hard drive, among other things, he said on Friday.

Microsoft has released a patch for the original hole, which was reported about a month ago, said Stephen Toulouse, security program manager for Microsoft's Security Response Center.

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