Hi MARIO 58, DennyM, Serendip,
At least with MARIO 58 and Denny M, I spot what could be a common link between two different OS's and systems.
When I install or upgrade an important security program such as Avast!, I make it a point to turn off (disable) any other security programs that provide real-time protection before attempting to install, and only re-enable this sort of program when all appears to be clear and well.
Reason is, programs that actively scan your system will allow the install, but during the rebooting process, Avast! files are replaced with new, and if certain features in these other programs are enabled, they then do not know what to make of these changes, block them, and that action will sometimes lock up a system.
I think ZoneAlarm, and possibly SUPERAntispyware Professional, might fit the bill here. Maybe, maybe not, as I am not in front of your systems. Just a thought.