I've noticed a few posts recently on RC and other forums where people have had trouble logging in to password-protected sites, and one where someone couldn't communicate with an online printer, and in each case the cause was their virus checker.
To most people, if an AV program checks for viruses, and blocks intruders, then that's all they think they need. What they should also ask themselves is....
Will it significantly slow my computer down ?
Will I get any nasty surprises when suddenly I can't log in to my printer, or access my favourite sites ?