I've used avast for several years. I have NEVER had a problem with it. In FACT, I have convinced several people to trash Norton and McAfee in favor of avast.
It auto updates, the licence is free if you register, (which is also free), it is FULL FEATURED. This is not shareware garbage!
It does require the user to be a "home" and "non-commercial" user. Otherwise, they have avast pro. Which is even better then avast 4 home, which IMO blows the rest away.
They don't win 100% Detection awards for nothing!