Quite often when you are doing a build of a large project, the build will fail when some VS utility returns an error. The problem is that you can't see why it fails.
A useful technique is to use the VS command prompt (Start - Programs - VS 2005 - VS 2005 Tools) and cut and paste the command line that failed from the VS output window.
Then execute it - you'll see all the errors.
The other option is to select Tools - Options - Projects and Solutions - Build and Run and change the build output verbosity to "Detailed" or "Diagnostic". Just change it back afterwards or you'll be swamped!