Unfortunately, you don't. There are password cracking programs and procedures, but typically, reinstalling Windows is the solution.
You're off topic for this forum, which is dedicated to Windows Live OneCare. You may want to try the Microsoft Public newsgroups here: http://www.microsoft.com/communities/newsgroups/list/en-us/default.aspx
-steve
Microsoft MVP Windows Live / Windows Live OneCare Forum Moderator