After you have installed the service pack, you do not need to reinstall it if you add a device or new software to your computer. For example, Windows 2000 and Windows XP include a file named Driver.cab, which contains all of the files potentially installed by Plug and Play (PnP)-class installers. Setup and other components use this file to install the drivers for new devices (for example, a PCMCIA card or a USB keyboard) without requiring access to the Windows CD or the network.
The service pack does not update the Driver.cab file itself; instead, the Update.exe program installs an additional driver file named Sp1.cab (or other if the SP number is greater). This file contains only updated versions of drivers that exist in the original Windows 2000 or Windows XP Driver.cab file. In addition to the Sp1.cab file, the Update.exe program installs a new Drvindex.inf that points to Sp1.cab for all of the updated drivers and also points to Driver.cab for all of the remaining drivers.
Update.exe installs a Layout.inf file. Layout.inf ensures that the correct software is installed and that all the correct updates are made to Windows 2000 or XP when you remove services from or add services to your computer. For example, if you remove a service from Microsoft Windows NT 4.0 and then reinstall it there, you must reinstall the service pack after you reinstall the service. This is unnecessary in Windows XP. The Layout.inf file is updated automatically to determine which files must be installed from the original Windows CD or from the service pack and, if necessary, the file prompts for the appropriate media. When you install a Windows 2000 or XP service pack, the Update.exe tool performs the following actions:
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.
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