With some Microsoft operating systems such as Microsoft Windows NT you must re-apply the latest service pack after you make a change to your computer. Although you do not have to do so in Windows XP, there are a few situations in which you must reinstall SP1 for Windows XP.
Reinstall SP1 for Windows XP in the following situations:
- After you perform an in-place upgrade (reinstallation) of Windows XP.
- After you perform a repair of Windows XP.
- After you use System Restore to restore your computer to a restore point that was created before you installed Windows XP SP1.
- After you upgrade from Windows XP Home Edition to Windows XP Professional Edition.
News source: Bink Windows XP