There's no real easy solution to this, but here's what worked for me:
Update from directory.
- Get the application update from http://free.avg.com/us-en/download-update (the first Windows link that indicates "All Necessary Modules") and remember where it is!
- Open AVG Anti-Virus console, Tools, Update from Directory
- Point it to the download folder from the first step. I told you to remember it! :)
You should now be able to successfully update AVG without having to reinstall.