The pirated copies of the software, which are circulating on various networks, are infected with a Trojan downloader embedded in the setup.exe file.
Trojan-Dropper.Agent, or Trojan.Agent, can take control of the host computer and add it to a botnet, with the potential to cause system failure. It can also result in identity theft or financial loss for the user, according to Australian security vendor PC Tools.
The news comes just a month after criminals released pirated versions of Apple's iWork '09 office software suite containing spyware onto peer-to-peer networks. Back in January, an early malware-infected build of Windows 7, labelled 'Windows 7 Ultimate Build 7000', was also leaked onto the internet.
Rik Ferguson, solutions architect at security firm Trend Micro, warned users to only download software from a trusted source.
"It's all about credibility," he said. "Pirated copies of stuff found on torrent sites are very low on the credibility scale."
The genuine Windows 7 release candidate is available for download from Microsoft's official Windows 7 web site.