Now that I have an Internet connection again, I decided to fix the project files that you guys have had problems with. It seems Visual Studio uses relative paths for everything, except the XNA content assemblies.
Download Source Code
This release is exactly the same as the last one, so don't bother downloading if you've already fixed your project files.
Microsoft have recently released an update for XNA, which I will be checking out tonight. It has font support, although I gather it's fairly basic. I will try to integrate this into the GUI, although I suspect I will run into the same problems I had before with their font sample code. In case you haven't read my comments from a while ago, the problem is that anti-aliased fonts simply won't work with the GUI at the moment. The only way I can see it working is to implement multiple passes when rendering the controls. Personally I think that small font sizes should be used anyway, which look better without anti-aliasing.