"When you right click certain files, you'll see a selection to "open with," and when you click it, you'll see a list of programs to choose from. You can remove some of the applications from this list if you will never use them to open this type of file, or you can clear the list completely, by editing the registry.
Open the registry editor. (Start>RunRegedit will do it)
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).