This is a follow-up to my previous post.
I did a virus definition update to Avast 2 days ago. I didn't use my computer after that and had is shutdown subsequently. No problems detected, everything fine here.
Turned on my computer yesterday morning, surfed the net as I usually do, and BOOM! a blank screen and a reboot after using my computer for something like 10 minutes. This problem repeated itself 3 more times as the day progressed. Practically, I couldn't do anything with all these reboots. I tried disabling the Avast service too, but that didn't help either. The computer bombed out some 15 or 20 minutes later after reboot.