The site that I linked to above has a bunch of different scripts that repair various asociations. Just do a search for something like "doug knox repair associations" and it should be the first thing that comes up. I do not trust registry repair tools. I have seen way more systems broken than helped. The registry is a complex place, and a program needs to consider context when deciding that something is good or bad. It is really hard to create a program that does that accurately.
What you might want to do at this point is do some scans of your system and see if anything else pops up. Usually when you start discovering missing file associations, there is an underlying cause. Your system should not be resorting to a backup of the registry. That could be anything from an imminent disk failure, to some sort of malware present. Chances are that here are more associations broken that you are not yet aware.
Follow steps 3-6 and se if anything shows up. If it does, then you will want the assistance of the HJT team.
EDIT: Malware is a catch-all term for any type of malicious/undesired software.