Would just like to point out that the most dangerous malicious programs remain hidden at all times (don't appear in taskman, don't slow down the PC, etc.). They're the ones that open backdoors (including through firewalls), and/or log things.
Not having a problem doesn't mean you're PC isn't compromised.
However, too much faith in firewalls & antivirus packages is as dangerous as not having one at all. You will be fine without either if you have NAT and ensure that you don't install anything on your PC - ever (even the stuff that can arrive without your knowledge).
However, the general home user has absolutely nothing of value. That removes the need for a firewall (breaking into a PC won't get you much). The only thing a home user has is a potential zombie PC, or credit card details. Both of which require the user of that PC installing some malicious software. Firewall won't help in either case.