How to define 64 or 32 bit version of Photoshop is installed on computer?

If you are a 64 bit app you can see both the 32 and 64 bit parts of the registry. See the WOW64 section for the 32 bit entries. If you are a 32 bit process you should only see the 32 bit side of the registry.

The interesting keys would be here

HKEY_LOCAL_MACHINE\SOFTWARE\Adobe\Photoshop\12.0\

HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Adobe\Photoshop\12.0\

If you can find the second key then you are a 64 bit machine and the wow6432 is where the 32 bit version is and the other key is where the 64 bit version is. If you only find the first key you are 32 bit version and that is where the 32 bit version is. This is in theory as what happens if a user only installs the 64 bit version.

What I do is...

If I want to launch 32 I run cscript out of windows\syswow64. The 32 bit version of cscript.

If I want to launch 64 I run cscript out of windows\system32. The 64 bit version of cscript.

Which is pretty funny given the names of the folders!

The type of user running the program of course plays into this as well. I am an Administrator and have that pesky 'are you sure you want to do something' dialog boxes OFF.

I also think you can look at some environment variables to see if you are 32 running on a 64 bit os. But I have not played with those much. Here are some interesting ones from my machine.

PROCESSOR_ARCHITECTURE=AMD64

ProgramFiles=H:\Program Files
ProgramFiles(x86)=H:\Program Files (x86)
ProgramW6432=H:\Program Files