From this site:
http://www.castlecops.com/t100412-How_to_Manually_Update_AVG.html
"If you need to perform a manual update first you will need to get the latest update files from the AVG Free website at the AVG Free Update Files
The Priority updates contain the full virus database.
The Recommended updates include the database and new program updates.
The Optional updates include those plus additional files that may be of help.
Choose the one that applies to your system, if in doubt, get the recommended or the optional update file and download the file to a directory on your computer that you can find later. You must remember where this is because you will have to tell AVG where to find these files later.
After you have the files,
1. Verify and update your computer's time and date, AVG uses this info to determine if you have the correct files.
2. Open the Control Center,
3. Double Click on the Update Manager or select it then press the Properties button at the bottom of the screen.
4. On the Properties screen unselect the option labeled "Do not ask for the update source", click Apply and Ok.
5. Now click the Update button on the bottom of the screen.
6. In the pop-up window, click the Folder button.
7. Navigate to the folder where you had downloaded the update files into and click Ok.
8. AVG will update now with the file you had selected unless it contains the same as you already had.
That should be all you need to have updated your system.
Note: Manual update is generally not recommended as it requires that you download the big full-update file. We strongly recommend to use the automatic update if at all possible as it downloads only the differential update file that is typically rather small (1/10th or less of the full update)."