WGA Problem

Ok..i recently had a friend put xp pro on my computer. So he did that and when i go to download the new wmp it doesn’t work. Windows says the key is invalid. So when i try to find keys to put none of them work. I saw something on this site where i go in to system32 and delete a file or something like that. I could not find a such file that they listed. If anyone could help i would greatly appreciate it. Thank you