Windows XP Service Pack 3 has been cropping up on numerous file sharing and developer sites over the weekend.
The 370MB software update is exciting interest in the developer community because it contains some important fixes and enhancements.
Microsoft has not confirmed whether the file resembles the final release of SP3, which is due in the first half of next year after having been delayed several times.
The package contains fixes for hundreds of flaws that have already been patched, as well as software for improving the networking capabilities of XP machines and other changes to the look and feel of certain operations.
It is believed that a version of the Service Pack was sent out to a few developers under strict non-disclosure agreements this summer.
The leak, if confirmed, will be an embarrassment for Steven Sinofsky, senior vice president of the Windows and Windows Live Engineering Group, after he has been so keen to improve the security of test code.