Here is AVG’s page which includes the AVG removal tool:
http://www.avg.com/us-en/utilities
A good command line for it is thus:
removeavg.exe /norestart /skipask /silent /deletedirforcehard
You’ll want to replace “removeavg.exe” with the current name of the executable. Also, you will still need to do manual removals of services, possibly toolbars, etcetera; it does not get everything.