Avast is an antivirus program. Norton has antivirus protection. Running two antivirus programs on the same computer is bad. They will interfere with one another, and cause various strange symptoms.
Antispyware programs are different. You can run an antivirus and an antispyware program at the same time without problems. An example of an antispyware program would be Spybot Search & Destroy. It is a different type of program.
I suggest that you disable one of the antivirus programs and see if it helps.
P.S. You should select and install at least one antispyware program.
I have a Dell 4700 running XP that had a trojan that was saying I needed to install spyware software. Norton wasn't taking care of it so I installed avast. After I installed avast it wouldn't let me log into the any of the user account. I would click on a user account and it would act like it's loading and the desktop would flash then it would all of sudden log out the user and say saving profile settings. Then it would just go to the log in screen with the list of user accounts. I started it in Safe Mode with networking and was ablt to log on. I ran an avast scan. As the scan was running it said it needed to shut down and restart the computer and run the scan on start up. I ran the scan and had it delete any problems it found. It had 1712 infected files. After it was done I restarted the computer and tried logging on and it's still not loading the user account. I tried starting it in safe mode again but this time it wouldn't load the admin account. It acted like it did and the it would log off all of sudden. I tried all of the safe mode options and last known good configuration but none of them will let me log on. Any ideas?