Avast and those Nvidia drivers work fine. So it is not just that combination. Could be in combination with other drivers/hardware. Check the event and system log for errors. If there are any us www.google.com and the Microsfot knowledgebase to solve them. See if it still happens then.

Besides this here are other things you can try:

  1. Disable all applications from loading at boot time, leave only Avast there.
  2. Check the temperatures of your case, cpu and video card. (make sure ALL fans/coolers are clean)