The beta version of the OS, reportedly Windows 7, build 7000, can be downloaded from several infamous torrent sites often used to share pirated content.
Microsoft isn't planning to officially release beta copies of Windows 7 until its MSDN developers' conference in January. A final version of the OS isn't expected until late 2009 or early 2010.
Microsoft may not be overly displeased with the leak.
The company is urging partners to begin work on Windows 7 product development as soon as possible to avoid the sort of compatibility problems that plagued the launch of Windows Vista in January 2007.