The thing with this is that it's all dictated by the registry. So the only way to "reset" all file associations is to have had a backup copy. Other things you can do:
- Right click on the file in question and select "Open with..." then "Chose default program". If it doesn't show up there, then you'll have to click on the "Browse..." button to find it. Make sure "Always use the selected program to open this kind of file" is checked.
- Look in the registry for the file extension. If it's there, find another one that you know has a default program and copy what that looks like to the entry for the file extension you want.
- Make a new user account.