I've noticed that the file association combobox sometimes autocompletes itself. In my case, if I try to register ".s", when the combobox loses focus, it autocompletes to ".sol", which is a file type associated with some other program (flashplayer IIRC).
I had the same problem on Windows XP.
I fixed it with regedit.
Here's the steps:
1. Start->Run... -> regedit
2. Right-click on HKEY_CLASSES_ROOT
3. Click Permissions
[Strangely, "Everyone" was the only entry for me]
4. Add "Administrators" and tick all the "Permit" permission boxes.
That fixed it for me. Not sure why but give it a try. If in doubt, make a backup before you change anything.