About half of the downloads claiming to be free versions of Microsoft's Vista operating system are actually malicious Trojan horse software, security vendor DriveSentry warned Thursday.

With Vista's consumer launch just days away, hackers have been bombarding discussion boards with offers of "cracked" versions of Windows Vista, which are typically being distributed on peer-to-peer networks, said John Lynch, vice president of sales and marketing for DriveSentry.

These posts offer downloads of the operating system that skip Vista's activation process, created by Microsoft to prevent users from running illegal copies.

Users who fall for the scam can end up with some pretty nasty problems, according to Lynch. DriveSentry researchers have found malicious key-logging software and spyware on about half of the downloads it has examined recently, he said.

Pirated versions of Vista have been in circulation for several months now, and one Vista "crack," called "Windows Vista All Versions Activation 21.11.06," has already been identified as a Trojan. Trojan horses are malicious programs that present themselves as harmless or useful software.

DriveSentry has also seen criminals disguise Trojans as free versions of Windows XP in the past, Lynch said.

It's an effective technique, he added. "Someone that's stealing the software to begin with is not going to raise a fuss if the software turns out to be malicious."