When should I reapply a Service Pack?

A. A. You should reapply any Service Pack (and subsequent hotfixes) whenever you add any system utilities/services or hardware/software. A good rule of thumb is if the computer says "Changes have been made you must shutdown and restart your computer" reapply your service pack before the reboot.

The only problem is once you reinstall a service pack, unless you uninstall then reinstall, you will lose the ability to uninstall it.