get a free avg from here, not the licensed version, but it works. http://www.majorgeeks.com/download886.html
the problem with the serial number is that it will let you install the software, but once you update it through the server, then your serial number will be validated and it will complain that the serial has been used repeatedly using several IPs, so you will be rejected.
anyhow, here is a simply trick for you to do. reinstall your avg software, using your old serial number. then, go to the avg site http://www.grisoft.com/doc/72/lng/us/tpl/tpl01 and download the updates, save it on your harddisk. then, tell avg to look for the updates locally, not on the server. that way, your serial number will not be verified on the server but your avg antivirus data should be updated. hope this works. (i just thought of this while reading your post, not sure if it will work, but worth a try).