You're off topic for this forum, which is dedicated to Windows Live OneCare.
If your PC came with Windows pre-installed from a major PC vendor, the product key may be on a sticker on the back, side or bottom of the PC.
See this site for information: http://www.microsoft.com/genuine/default.aspx?displaylang=en
And, if the PC came from a major vendor or PC reseller, contact them for help in obtaining a valid product key.
-steve