You Do Not Have to Reinstall Windows XP Service Packs After System State Changes

After you change the system state by changing or adding Windows XP components, you do not have to reinstall Windows XP service packs.

When you install a Windows XP service pack, the Update.exe tool performs the following actions:

• The Layout.inf file is updated with an additional source for the Windows XP service pack files. When you change or add components, the Layout.inf file points to the correct source.
  
  The source location is stored in the following registry key:
  HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Setup\ServicePackSourcePath
  The source might be a network share for the Windows XP service pack, the Windows XP service pack CD-ROM, or if you install the Windows XP service pack by using the Express Installation feature from the Web, the %SystemRoot%\Servicepackfiles folder.
• The Drvindex.inf file is updated to point to an additional .cab file for drivers that are updated in the Windows XP service pack. The .cab file is named Spn.cab (where n is the service pack version number). This file is installed in the %SystemRoot%\Driver Cache\I386 folder.
• A new Spn.cat file (where n is the service pack version number) is installed for the updated files. This file replaces any previously installed Spn.cat file that was installed with a Windows XP hotfix. The Spn.cat file is installed in the %SystemRoot%\System32\CatRoot\{F750E6C3-38EE-11D1-85E5-00C04FC295EE} folder.
• Windows XP service packs also perform version checking on files so that post-Windows XP SPn hotfixes (where n is the service pack number) remain in place. You do not have to reinstall hotfixes after you install the service pack. For example, when you install Windows XP SP1, hotfixes that you installed before you install Windows XP SP1 are updated to the Windows XP SP1 files. Hotfixes that you install after you install Windows XP SP1 are not replaced.

Note that hotfixes do not update the Layout.inf file. Therefore, if you install a hotfix and system state changes occur, you may have to reinstall the hotfix.