First of all. How big a harddrive have you got? When was the last time that you did a defagmentation and a clean up of it? You can also try turning off your firewall as well, as AVG 8.5 and 9 may not install on your system with your firewall running. Also what programe are you currently running, XP Home Edition, XP Pro, Vista, or 7?. I know that AVG will not run properly on Windows 7 as I have tried it and I found that I had a really bad time trying to get it to run properly.
You can try a disk clean in the property section of you harddrive, but you will have to restart your pc and let it run its course. It depends on how big your hardrive is and how much you have on it to how long it will take to finish. I do mine once a month and I have no trouble with this pc that I am useing and I am useing XP Home Edition on this one, but I am running Windows 7 on one of my other pc's and still do a disk clean up once a month on it.
I hope that I have helped in some little way and I wish you luck. Let me know how you get on with it as I am interested on how you get on.