Sometimes the updater does not uninstall the older version as it should.This can be caused by different instances.
Try this first.
control panel > admin tools > services and locate the AVG service and stop it from running. Also stop any other AVG services such as the avg auto updater...they will have the AVG located within the service.
Now go back to the add/remove application and uninstall the latest version and then the older version and then any updates that are installed for AVG.
Once that is done:
open my computer > local disc /C or which ever partition it is located on in your computer then navigate to the programs files and look for any AVG folders. If there are any delete them.
Then reinstall the latest version of AVG or any other anti-virus program you want to try!
Change Is Good