Επειδή την έφαγα ψάχνωντας πολύ ώρα και δεν το ήξερα.

Do not repair VS 2008 SP1 from installation media

If you need to repair Visual Studio 2008 once SP1 has been applied or wish to change which features are installed, you cannot run setup.exe from the original 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."

Workaround

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.