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.
NOTE: When you upgrade from Windows XP Home Edition to Windows XP Professional Edition, Windows XP SP1 may be listed in Add/Remove Programs. However, if you try to remove Windows XP SP1 by using the Add/Remove Programs tool, you may receive an error message that states that Windows XP SP1 has already been removed. When you perform an in-place upgrade of Windows XP, you reinstall Windows to the same folder. When you do this, files that were installed by SP1 for Windows XP are overwritten by the original version of the files from the Windows XP CD-ROM. This also occurs when you run a repair or upgrade from Windows XP Home Edition to Windows XP Professional Edition. For this reason, you must reinstall the service pack.
News source: Bink Windows XP
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