You would have been best to follow through and install avast5 version 5.0.418 (effectively a clean install).
Now you have to download avast5 and save it to use later.
Download the avg removal tool and save that for later.
Uninstall avg and reboot.
Run the avg removal tool and reboot.
Install avast5.
- AVG8.x (or higher) Remover, download tool from here,
http://www.avg.com/download-tools there is a 32bit and 64 bit windows version, ensure you use the correct one.