I programmed a simple, little game with wxpython and created an .exe of it with py2exe (and in a 2nd try with pyinstaller). I also programmed an installer which just creates a folder and copies all the game files into this folder. I also created an .exe for this installer. Everything works fine on Windows 7. But on Windows XP i get this problem: My installer.exe succesfully works, but when I start the .exe from my game then I get this error:

foo.exe is not a valid win32 application

I use python 2.7
Is the Windows 7 box you're using to create the installer 64-bit? Is the Python version you were using on Windows 7 64-bit? If so, then you'll need to create the exe using a 32-bit Python, 32-bit wxPython and 32-bit installer. 
A 64-bit program will run just fine on a 64-bit system, but it won't work on a 32-bit one. However, a 32-bit program will run on both.
py2exe/pyinstaller: execution failure