Just upgraded a client from AVG 9 to AVG 2011. This was to the Business Edition of AVG Internet Security (and Anti-Virus) 2011.
One of the laptops running Windows XP SP3 failed to uninstall the previous version properly then screwed up the install of the new one.
When you opened the AVG Console, the Firewall reported an error 'NDIS driver not found'.
Running a repair install didn't help and that article about manually installing the AVG Network Filter Driver from Local Area Connection properties didn't work either.
I did an unistall of AVG and was still connected to the Internet and network ok, however when I looked in Network Connections there was no Local Area Connection and no Wireless Connection.
To fix that I went into the Device Manager and uninstalled both LAN and WLAN devices. Then did a Scan fro Hardware Changes to reinstall them. That did the trick and AVG 2011 reinstalled successfully after that, presumably finding the NDIS that had been MIA.
Rather than doing a full AVG uninstall and reinstall, maybe worth testing if uninstalling and reinstalling the LAN and WLAN devices recovers the NDIS error ...