So, your problem is thus: You have an eval copy of XP-64 that has expired, and is now making your life miserable. It has offered that you purchase it, but you just want to dispose of it and go back to normal XP?
Do you want to keep XP-64? I think MS had some sort of a program that allowed you to relenquish an XP (32 bit standard) key and recieve a 64 bit key. (just looked, and the program ended july 30th). If you are really serious about keeping then you'll have to buy a new key.
If you just want normal XP back:
Unfortunatly, if I understand it correctly, XP-64 is a totally dirrerent entity from XP. You are going to have to back up your data and reformat. If you have a copy of XP, then you should be able to use that key again, if not, just call up Microsoft and eplain your situation, and ask them to give the activation code manually.