This annoys me every time. You go to build on a new PC and get some nonsense like this:
The install location for prerequisites has not been set to ‘component vendor’s web site’ and the file ‘DotNetFX35SP1\dotNetFX20\aspnet.msp’ in item ‘.NET Framework 3.5 SP1’ can not be located on disk. See Help for more information.
The answer is very clearly in the Visual Studio 2008 SP1 Readme. And it works. Oy vey.