VS Action can hang

I have  a VS2003 application that I've been building happily with FB but for some reason (yet to be identified), I am now getting lots of Compiler Error CS0234 - The type or namespace name 'name' does not exist in the class or namespace 'scope' (are you missing an assembly reference?) errors (for things like Windows, Helpers, Notify etc..).

If I change the 'Configuration' from the one I want then it will compile, but the one I do want does compile withing the IDE itself.

I then wondered what difference 'Use environment variable for C++ paths' would make (thinking maybe I was missing a path or something somewhere), so ticked it, and the action hangs becasue it give a list of the command line switches because 'useenv' in an unknown switch.

Not a huge issue as you can stop it, but thought I'd mention it. Mind you, any help on the initial compiler error would be welcomed...