Actually its not AVG thats the issue.
Set the system to show hidden files and folders as per:
http://www.cyberanswers.org/articles/show_files.htm
Then right click start | explore | c | windows | repair
Then right click on autoexec.nt and select copy
Now in the explorer window click c | windows | system32
Once inside the system32 folder click edit then paste. Accept any prompt to overwrite any file..
Then reboot and retry.