aha...the other way you can do it is open any explorer window (such as My Computer) and then go to Tools->Folder Options.
There choose File Types and wait for the list to load. There you can select any file type and change the program, add new file extensions and associations to the list or even delete anyone.
If these two things don't work, just go to help and support and then type in file association and it should come up with the relevant article.