Hi
The trouble with all windows operating systems is the registry gets bloated overtime, and if you google about win rot this is very interesting subject. I reformat XP at least once every 2 years or so and the speed increase is amasing! on a new install of XP. Im sure that you have some kind of registry fault that is preventing avast from working correctly as it sometimes happens on booting. Have you got any restore points at a point as when you first got the computer and the only snag is it may restore the old virus program entrys that you have removed previously. Have you got windows defender installed as this could cause avast trouble?