Registry is really the way to go with this one... run 'Regedit' and back it up then...
Go to the following key: HKEY CURRENT USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts.
Scroll down the list of file extensions to the one for which you want to edit or clear the list.
Click the + to expand OpenWithList.
In the right pane, you should see a list of programs (for example, Word.exe, notepad.exe, etc.).
Click the one(s) you want to remove. Press the Delete key.
If this doesn't remove the program(s) you want to remove, go to: HKEY CLASSES ROOT, scroll to the desired file extension, click OpenWithList, and remove the program there (the settings under HKEY CURRENT USER apply only when your user account is logged on. The ones under HKEY CLASSES ROOT apply to all users).