There is only one solution that I have found for this, uninstall and re-install the program. It gave me endless nightmares until i did that, it started this morning and eventually i took the drastic step and it worked. :D
That is the fix for almost any problem with AVG and a number of other programs as well.
If your AVG has been installed for several months, you need to go to the download server for your particular version (FREE or Retail) and download the very latest version. It will contain all of the latest fixes and updates.
Then do a clean install after completely UN-Installing the old version and removing the folder for it on the HD.
I recently had a situation where the email scanner was not active. I had to go into the settings for it and add the pop3 and smtp addresses for GMail and then it worked OK. I know....that's something that would be beyond the ability of a casual user.