I had a problem with system shut downs and Firefox when I had one version of AVG anti-virus.
I changed to avast! and the problem went away.
I am running Firefox 3.5.4.
I am running Firefox 3.5.7 on XP - no problems.
Does avast take up less space on computer than AVG. Loved AVG until I tried version 8. Slowed things down a lot. Installed Norton.,but do not want to renew it. Looking for a free system.