Do not repair VS 2008 SP1 from installation media
When you run repair from media you may see an error like, “A problem has been encountered while loading the setup components. Canceling setup.”
Open the Control Panel and go to Add/Remove Programs, or on Vista click on the “Uninstall a program” link under Programs. Find Microsoft Visual Studio 2008 (the exact product name will vary based on edition and language installed) and click on the Uninstall/Change button. When setup launches and components are loaded, you will be asked to repair, change features, or uninstall. Choose the option you which to perform and continue as directed.
Visual Studio is actually a collection of different installation packages, just as VS2008 SP1 is a collection of different installation packages. VS2008 SP1 replaces some of the original packages using minor or major upgrades and patches other products. This changes information about the products originally installed like the ProductCode and/or ProductVersion. This affects the detection logic that setup.exe uses to determine what to do when installing, repairing, or uninstalling the product.
This information is stored both on the installation media as well as on your hard disk under the target installation directory. For various reasons including that media is often write-protected, we can only update this information stored on your hard disk. So once SP1 is installed, the detection and package information on the installation media and on your hard disk are out of sync and only the copy on your hard disk contains the correct information.