Windows 7?
You can temporarily activate your version without a valid license key by running the command "slmgr.vbs /rearm" from an elevated command(dos) prompt.
Just click the windows button, in the search field type "cmd", but don't hit enter. You'll see the CMD app appear in the found search. Right click the app and select, "run as administrator". Click yes to the confirmation prompt.
At that black dos prompt just type "slmgr.vbs /rearm" - hit enter, let it run - I think you get some kind of confirmation that it was successful. From there reboot, and you're good for 60 Days. You can only run this 3 times though before it demands licensing.
Blam, you're welcome.
So you don't think this is some shit to screw up your computer:
http://technet.microsoft.com/en-us/l.../ff793433.aspx