I had Vista 32-bit and 64-bit freezing issues on my Laptop, all when using Firefox on certain websites. Had Web Shield Firefox plugin disabled, didn't matter...
I wasn't even aware it was Avast until I had replaced EVERY major piece of hardware in my laptop, being completely convinced my 1 year old daughter knocking it onto the ground had caused physical damage, when it turned out that reinstalling Vista due to Hard Drive damage was what caused the start of my hard lock/freezing woes (Due to installing Avast to a version that was buggy, probably 2223).
The symptoms are sudden, total freezing, screen remains intact, no mouse movement, hard drive activity gone (HDD indicator not lit solid either), Wifi Adapter button non responsive, complete system freeze, also known as a hard lock.
Doesn't matter how much free RAM you have, or the CPU temperatures... Always seems to be some combination of running two or more tabs with certain websites, for me it would always be wunderground or CNN.com, but I could use gmail and youtube for weeks without freezing. Also got freezing when giving feedback on Ebay purchases, but other than that Ebay worked fine without freezing.
I figured it out when I tried to get the system to lock when using Firefox on those websites that cause the lock ups, BUT while Avast was disabled... And NO locking!
I have since downloaded and updated to the latest 2016 Avast and with all shields active, so far so good!