These are a few things you could try. Not sure if they will work. Some times a program needs certain prerequisites for it to work such as .NETFramework 3.5, etc. Maybe AVG's upgrade conflicted with one. Just guessing.
Have you run Windows Updates? Are there any outstanding service packs?
Try going to Programs and Features (Win 7) or Add/Remove Programs (Win XP) and right click on AC and run repair.
If this does not work, simply try uninstalling AC and reinstalling.
If unsuccessful, uninstall or turn AVG off. Uninstalling would likely work better. You may need to install AC again.
Run System Restore. If this fixes AC, you may have a non-upgraded AVG, but that would be something you could live with.
Is Windows Installer service running. You can try restarting the service. I don't think re-registering it will help.
Just some ideas.
