Lost product key on Windows XP Pro. x64…

I had Windows XP Pro. x64 on my computer for awhile when I moved from Vista, Now I am on vista again, and I have about the same problems again, cause I do lots of stuff on my computer like video and audio production and I run a radio station on this computer… and I want XP back… I lost the product key. I backed tracked a thoudsand times, and called Microsoft, they didn’t really help, cause I told them I don’t have some other key they needed. I tried to tell these support newbies that I got this from the site last year. Would anyone know how I can get the key? Like may be a file in the CD I can look for?