I assume you are referring to having to reactivate XP. every time XP starts there are ten things it looks for. 1. Processor model 2. Processor serial number 3. Ram size 4. graphic adapter 5. IDE controller 6. SCSI host adapter 7. Harddrive 8. Cd Rom Drive 9. Volume serial number 10. MAC address. as long as seven of the above are found to be the same windows will not require reactivation. (except of coursing changing a hard drive if windows is reinstalled on a new hard drive obviously it will have to be activated.)
Also.....once you change one of the above....say CDrom, you can then change it as much as you like it wont count against you. once the original item is gone windows doesnt care how many times you change that item.
there is a nice little utility called XPInfo that will show you how windows views your computer( regarding how many changes have been made since activation)