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64bit of consumer hardwares is usually "amd64" architecture which can run both 32bit apps and 64bit ones natively. Windows Vista 64bit edition supports both 32bit and 64bit system libraries, so basically you can run both type of applications as well. (Note that IA-64 architecture does not allow this.)
Compiling a program in 64bit is not much related to the platform that the compiler runs. But, of course, to run and test the result binary requires the corresponding architecture.
As many mentioned above, VS2008 let you choose the target architecture, so there's no problem.
I've found that just the setup.exe created by Visual Studio 2012 Express won't work on XP, but if you go ahead load MS 4.0 .NET Framework from the Microsoft Website then the *.application will load and install without using the setup.exe at all.