I ran across an interesting problem the other day, a client for some reason had mistakenly associated a particular file extension with a program, which is rather easy to do in Windows 7, causing them all kinds of problems. My client needed desperately to unassociate the file type for the project they were working on. My experience had reminded me that there was no method in Windows Vista or Windows 7 to unassociate a file type, only link it to another program, without digging into registry. Odd enough as that was, I did not want my client digging in the registry to solve their issue.
After a quick google session I found a small and very simple GUI tool written just for this issue. The Unassociate File Type Utility (VirusTotal Report) written by Ramesh Srinivasan is perfect solution. Allowing the user to select the file extension from a list and remove the association for that user, or if proper privileges are granted, the machine altogether.