After installing Visual Studio 2008 SP1, if you attempt to add or remove any Visual Studio 2008 features via the add remove programs in windows, you receive an error similar too "A selected drive is no longer valid". I encountered this when attempting to add x64 compiler for c++. The only information I can find on this is at: http://social.msdn.microsoft.com/Forums/en-US/vssetup/thread/2f3d0378-3175-49ae-acb7-012594a1bf3c
This suggests that you must uninstall SP1 prior to making any changes to the install. This dose not sound like a solution in my opinion.