I installed AVG 8 in place of Comodo Antivirus as Comodo Antivirus is being discontinued in favour of a free integrated suite, which the helpline people at Comodo stated required you to uninstall Zone Alarm and use their security package with it. I was not keen to scrap Zone Alarm in favour of an untried security suite.
AVG 8 created cross linking problems between icons, and multiple Registry errors.
I later tried installing Comodo Integrated Security [the free version] on a spare computer and replaced Zone Alarm just to check out their security system.
During the process I discovered it actually does allow you to install the Antivirus portion only without replacing Zone Alarm, despite their helpline saying otherwise.
So, after unloading AVG 8, and running several Registry clean ups with re-booting required after each, I am now running the Comodo Antivirus component with Zone Alarm here. A full virus scan on this computer revealed only a marginal spyware threat that had not previously been detected, although it has been on the computer and passed scanning before - felix22.exe - a cute little cat cartoon character. I also had to turn off System Restore to stop the problems recurring after re-booting.
I don't know whether this is a fault in AVG's design or whether the AVG download site has been breached, but I though a warning was appropriate.