If the PC you intend to format and reinstall XP onto has already had this serial number used on it, you can still use the serial number on that PC.

Micro$oft allow 3 major hardware changes to your PC before you have to contact them to get your serial key activated. As long as you are under this limit, you will be fine re-using the key.

If you have exceeded this limit, all you hav to do is call Microsoft, explain that you have changed the configuration of your PC. They will give you an activation code that you input on your PC, and they reset your number of activations on their system.

Buying Vista because you have no XP key is daft, as you would pay less just buying another copy of XP!.

In any case, Windows 7 will be out in Q4 this year, so I would not advise buying an operating system till then.

If you need further info, PM me