Click Start>Run>type in regedit ENTER
Look for the following entry;
HKEY_LOCAL_MACHINE \ SOFTWARE \ MICROSOFT \ WINDOWS \ CURRENTVERSION
Click on the CurrentVersion key in the left pane of regedit, and in the right pane, scroll until you find the value ProductKey. This is your CD Key (not to be confused with ProductId which is the number Microsoft assigned to you when you registered Windows. This is for Retail Versions).
If this is an OEM version, the key will have OEM in it and MAY be called ProductID rather than ProductKey. Write down both values if you are unsure.