If you are still experiencing this problem later today try the following to manually update the defs.
Go to http://free.grisoft.com/doc/1 and click the AVI update link, download the latest .bin file and then launch AVG's control center.
In the control center, right-click on the update manager and select Update. It should prompt you to select a source (internet or folder, unless you disabled that choice in the past). Select folder and show it where you saved the .bin file you just downloaded.
If you don't get the option to choose a folder location to update from then Right click the update button and choose properties and then untick "do not ask for source" and then try again.