Answer .. You do not have a clue, do you? You just think free must be best!?
It isn't.
NAV is totally re-written since 2009 .. it's excellent.
AVG free is complicated to control manually, uses more system resources now than ever. It also did not stop Fake Worm Antivirus alert installing and wrecking a friends laptop, although I tested it with NAV 2010 which stopped it instantly.
Avast "paid for", ESet "paid for" and NAV2010 are fine. My NAV is currently using 18Mb memory in total, which is a few megs more than Keyboard software and 2/3 of what explorer is using.