Visual C++ x64 woes

[SweetCorn]

Hello all!

So I was pointed to this place after asking in the WinApi forum. I use Visual Studio 2008. I made a windows form program that works perfectly under the 32bit compiler. But when I changed the platform to x64, these things stopped working (they don't do anything):

MessageBox
CreateWindow
DialogBox

And when I call GetLastError, I get 193 (0xC1) which, according to MSDN, is:

ERROR_BAD_EXE_FORMAT
193 (0xC1)

%1 is not a valid Win32 application.

I have Windows 7 professional 64 bit edition and an x64 architecture for my CPU.

My suspicion is that I only have the 32 bit header files for WinApi.

Can anyone enlighten me?

Thanks in advance!