I'm having a problem when I try to build my solution in C# Express 2008. I need to build it for 32-bit architecture, but it always build for 64-bit. In Visual Studio 2008 I can choose the architecture, but I can't find this option in C# Express.
Is there a way to do this in C# Express?
Have a look at what the differences look like in the project file in the full Visual Studio, and hand-craft the same edits to your C# Express project - VS will respect those changes, even if it won't let you make them from within the IDE.
EDIT: As Jeff points out in the comments, if you go to Tools -> Options, tick the "Show all settings" box at the bottom left. Then under "Projects and solutions" tick "Show advanced build configurations". That will let you edit a lot more build options, including the target architecture. You may need to go through the Configuration Manager and create a "New Project Platform" however.
Even when you've done this, bear the first paragraph in mind for other options which simply aren't available within C# Express: it's still using the same build engine (msbuild) underneath, so as long as it isn't using any tooling specific to more powerful versions (e.g. the static checker for Code Contracts, only available for Premium and Ultimate) you should be fine.