You can reformat and re-install a PC till the hard-drive is dead, which is no set number really. Let's say 3-4 years. You will have no issues re-using the same XP key and disk on the same computer either.
If you go online to the hardware vendor's web site, you can find out any driver details you need. If it's running now on XP, it can run on any XP version be it Home or Pro or Media Center (32bit that is, if you install 64 bit, you need to find different drivers).