If you have XP on disk (LEGITIMATE COPY), you won't have any trouble. You'll have to do a new install, and to complete the activation process, you'll have to make a phone call to Microsoft. Or, you can use a BOOTLEG copy and reinstall every 30 days. (for about 2 years, at which time you will be TOTALLY locked out.)
"To activate Windows XP over the telephone, you can simply call a toll-free* number displayed on your screen. A customer service representative will ask for the installation ID number displayed on the same screen, enter that number into a secure database, and return a confirmation ID to you. Once you have typed the confirmation ID, the activation process is complete."