If the above statement applies to you, it is likely there is a solution to get your product key back. The following is one of quickest ways to recover it back.
Image of Jelly Bean Keyfinder to locate your missing XP Product Key
Step 1: Have handy the particular name of product. e.g Window XP/ps2, etc.
Step 2: Download and unzip product key finder here http://www.mjbforum.com/files/keyfinder.v1.5b3.zip
Step 3: Run and enter your product name
Step 4: Instantly your number will be seen on the bar displayed.