Is this the AVG anti virus or the AVG anti spyware. There is a known issue with the anti virus that affects a few computers if you install 7.5 over the top of version 7.1. Basically it doesn't autoprotect you. Solution is to delete AVG fully and reinstall rather than install over the top.
If its the antispyware quite a few of us have been having differculty in that it switches itself off. It also is a known fault and at the moment there is no solution to it as AVG have been unable to work out precisely what causes it.