Determining 64-bit vs. 32-bit Windows
You should be able to read the environment variable %PROCESSOR_ARCHITECTURE%. Here is some great information on it.
http://blogs.msdn.com/david.wang/archive/2006/03/26/HOWTO-Detect-Process-Bitness.aspx
Check out isWow64, I think that shopuld give you what you are after.
Determines whether the specified process is running under WOW64.
Jon's link is a total hack because it relies upon no new processor architectures being added.
See: http://support.microsoft.com/kb/556009 for the "official" batch file method.