Check to see if application is listed in the Registry's "Open With" key:
HKEY_CLASSES_ROOT\Applications
To add a registry key:
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts
Look for the file extension whose 'Open With' list you want to edit and find its OpenWithList subkey.
Credit to Preston Gralla, author of Windows XP Hacks (2005, O'Reilly Media), p. 112 for this suggestion.
Alas, you need to know how to competently ADD a registry subkey, and I'm not competent to advise you. But perhaps I've pointed you in the right direction.
To open the Registry, use either the command line RegEdit.exe or C:\Windows\System32\regedt.exe
As you probably know, it's a good idea to 'export' the registry to a flash drive or external HD so that you can recover a corrupted registry if editing goes awry. Also, create a Restore point in System Restore before commencing any Registry editing.
Rick/Portland, OR