Can't find product key for windows xp

Hello,
On approximately 9/15/2009 my roommate and me started checking for viruses, and spyware, ect. The report came back saying that I had 65 threats, so we ran different things and that number came down to 2,-1 being high risk. So we started deleting programs that had been ran (games, music, etc.) also deleting different toolbars because half of the page was taken up (msn, google, bing,etc.). In doing this alot of things were sent to the recycle bin. Evidently we deleted something of very much importance. Because when the computer was shutdown and started back up, it was asking for the product key. In trying to load windows xp, without this information I've found that I can"t... They're saying when I follow the directions that this copy of windows is not legit. My step-mother gave me the computer plus it's contents, therefore, I know that it's legit. However, the certificate of authentisity has been removed by someone and I can't find the product key. PLEASE HELP!!!!!!!!