When I right-click on a .reg file, I no longer have a "Merge" context menu option to select.
Double-clicking a .reg file executes the .reg file correctly (i.e., invokes regedit.exe).
I have attached a screenshot of what I see when I right-click on a .reg file.
I am running Vista Business x64 w/ UAC turned off.
Shift-right-click does not help, either (although that is not required to see Merge context menu).
I have not run any registry cleaners, and the system is otherwise working flawlessly.