My pc came used with windows already installed on it. Once I got the internet though I realized it was an invalid copy. I wasnt given the intalation disc when I bought it, but I now have a valid copy of windows xp that was given to me by a friend who now has a mac. The only problem is that I dont have the product installation key because for some reason he put the disk in a jewel case and lost the original one. Is there any way for me to get a installation key or am I going to have to spend a bunch of money for a new copy?

spend a bunch of money for a new copy!!
even if it had a key it might not activate ,because its been used before !

if its the O.E.M v you can only activate it so many times after that you have to plead your case with microsoft for a new product key, best to buy new xp o/s disk I did