It drove me nearly crazy yesterday, when I falsely associated a few files with the wrong application and could unsassociate them.
No mater how much I turned my registry upside down, how much I deleted, something was missing. I just couldn't find it. "F'k it!" I thought, and turned to google.
Lo and behold, I stumbled upon Unassoc. Simple, small, no installation! Oorah!
Simply search for the desired file type and click "remove file association (user)", after that Windows 7 will use the global file association. If you still have the wrong application as your standard application then do "delete file type".