Well, if I remember correctly, you should be able to download a .zip file or something of the virus def. updates. Could be wrong though...
David
EDIT: Yup, here ya go:
http://www.grisoft.com/us/us_updt7.php Just download the .bin file, and them copy that over with the AVG anti-virus, install the AV, restart windows, then install the updates. There should be an option within AVG to import udate file.