I've occasionally had to help users who frantically call to tell me that files they used to click to launch an application, no longer launch the correct application (or launch any application at all). After a brief conversation, I often discover they've been mucking about in Windows Explorer--specifically in the Tools, Folder Options, File Types tab--and they've changed the file type associated with the application. You can edit the registry to prevent users from casually changing these file type associations.

1. Launch Regedit.
2. Open
HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Policies\Explorer.
3. Change the REG_DWORD NoFileAssociate data value to 1.

This change prevents any user of the machine from changing file
associations through Windows Explorer. If you want to prevent only the
current user from making these changes, open
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\Explorer
and make the same change.