Wow! If it comes to buying and installing XP on an old computer, it almost pays for itself to buy a cheap but complete PC. Available now for about £270 complete with screen. Obviously, it is even cheaper still, just to get a base unit, but there are some deals today that will give you a new machine for peanuts, if you subtract the purchase price of the OS from the equation.
If you did that, you could maybe go Win 2K or Linux on the oldie.
Conan