Comodo Firewall...totally free, very secure, can see what ip all ports are transmitting to, has an AV built in. alerts you when progams make new directories (stopping trojans) so many great features.

AVG as a stand alone AV is very good.

So is common sense...don't download from stupid sites, limewire etc. don't accept files from people you don't trust etc.

I'm not sure if it's the best around but I use Avast and I'm satisfied with it.

Doesn't take up much space, excellent protection, doesn't shove a gigantic pop-up in your face asking for an update unlike others, it updates silently and informs you politely when it's done through a small window at the corner of your screen. and it's user-friendly.

Now I will go claim my paycheck from the makers of Avast, because I advertise so much.

EDIT: did I mention it's free?