Just go to this page:
http://www.grisoft.com/us/us_updt7.php
Find out which version update your current have. It's in the Information About AVG screen of AVG.
Then download the virus database .bin file.
Move the .bin file to the corresponding update folder on your computer.
I have Windows 2000 and it's located at:
C:\Documents and Settings\All Users\Application Data\Grisoft\Avg7Data\upd7bin
I'd suggest searching for the "Avg7Data" folder. You should find tons of those .bin files in the folder. Just put your newly downloaded update file there.
Then, run the Check for Updates. Select "folder" and then point to this folder. Ok.
It will find the update and do the updates.