At work I have two computers, one with VS2003 (the older one), and on the other one I've installed VS2005, they both share the same user-profile (network profile, I guess).

When building my solution I got this warning:
   CS1668: Invalid search path 'C:\Program Files\Microsoft Visual Studio\VC98\mfc\lib'
   specified in 'LIB Environment Variable' -- 'the system cannot find the file specified.'
This warning has occurred on each project build.

My first step was to try finding the path, which of course wasn't there.
My next step was to search the Environment variables and there it was the LIB variable was pointing on the given path.

As a conclusion, the VS2005 try to load the LIB variables at each build operation, and cannot find it.
Just delete the unrecognized paths from the LIB and your VS2005 will stop showing annoying warnings.

Moreover, this is a 1-0 to the VS2005, this is a good example for descriptive error.

* I've heard that this problem happen for others when installing the VS2005 on a computer with VS2003 that has been deleted.