Help Lost the product key

I have upgraded all of my computers with windows xp. Except for one. It isn’t new enough or compatable or something like that so it won’t take it. So I have desided to let it just continue on with Windows 2000 which is what all of them had. However, it keeps saying that I need to enter the 25 digit product key to reinstall windows 2000? I don’t even know where to begin to find it. Does anyone have the 25 digit product key for windows 2000?