If you have a 64 bit system, you can have both 32-bit and 64-bit versions of Photoshop installed (installing 64-bit version requires 64-bit operating system, f.ex. Windows Vista or 7). You can also install just one of them.
If you install them seperately (some people do it) , Windows will pick up the latest version to default the file association. What it means is that if you try to open Psd file from your Windows Explorer, it will launch the default (read: latest) version of Photoshop (either 32-bit or 64-bit). This brings some troubles – what if you want to open all the files in 64-bit but the latest one you installed is the 32-bit version? When you double-click on Psd files, they will automatically open in 32-bit version.
Here is the trick to change it (this will require changing the Windows Registry but it will be easy):
Open the Windows Registry by running regedit.exe (press Win key + R key to launch Open window)
Navigate to HKEY_CLASSES_ROOT\Applications\Photoshop.exe\shell\edit:
Change the path to the version of Photoshop.exe you want to use (32 or 64 bit), and that one will be the new default whenever “Photoshop” is chosen as a file association.
And that’s it! I hope you enjoy that and remember – Be careful! 🙂