As of Wednesday afternoon Pacific time, the BitTorrent search engine Mininova listed a 568.73 MB Windows XP Professional SP3 5503 file available for download. The file, which had 112 seeds and 417 leeches, had been downloaded 2083 times since it was posted Tuesday, according to the listing.

According to the file description notes, Microsoft will release the final version of XP SP3 on March 24, but the 5503 test build passes all Windows Genuine Advantage checks and is able to download Windows updates that previous XP SP3 builds weren't able to, which suggests that it could, in fact, be the final RTM build.

When Microsoft released Vista SP1 in February, the final RTM code was identical to the Vista SP1 Release Candidate (RC) 1 Refresh 2 test build that was released to a group of 15,000 testers two weeks earlier.

A Microsoft spokesperson, perhaps weary from the months-long barrage of Vista SP1 related inquiries, said the final version of XP SP3 is still on target to be released the first half of calendar year 2008, and the XP SP3 5503 build was a minor interim build released to address one specific issue for a subset of private beta testers.

"This build was not intended for public release, and anyone who has that build and is not part of the private beta is working with bits that Microsoft can't verify the integrity -- it's possible the bits may have been modified with malware or other bad code that Microsoft hasn't tested," the spokesperson said in an email.

XP SP3 was originally slated for release in 2006, but Microsoft has delayed it on numerous occasions since launching XP SP2 in August 2004. In February, Microsoft released XP SP3 Release Candidate 2 through its Download Center to users running PCs with XP SP2 installed.