I have AVG...it runs continuously. When you first download it, its set to run everyday unless you change the setting. When its suspects a page with risks on it...it wont even open. I think its run basically the same as AVAST.
Years ago i had Norton. It came with my computer that i had ordered. Then when it came time to renew it. It wiped out my computer entire harddrive. The funny thing is the EXACT same thing happened to my neighbor.