Turns out my file association was corrupt. Here's how to reassign the correct file association:
1. Click Start Menu, and then type regedit in the Start Search and press ENTER.
2. Navigate to the following registry branch:
3. Replace the [exe] with the actual file extension that you want to restore its file type association to revert back to original Windows Vista default. If you unsure, simply browse through all the sub-key under FileExts.
4. Delete the sub-key named UserChoice.
5. Exit from Registry Editor.
This worked after I rebooted.
Big, big thanks to "My Digital Life": http://www.mydigitallife.info/2008/06/22/restore-and-reset-file-association-in-windows-vista-back-to-original-default/