Firewalls.  They are the rage.  And for a good reason.  If you have a cable or DSL modem connection to the internet, you best be getting a firewall installed on your system.  Like yesterday, not next weekend. 
A couple I like are:
Zone Alarm  Free
BlackICE  $29-49 depending on where you pick it up

More Info

DOSHELP A guide to Denial Of Service (DOS) attacks, firewalls, and more.  Well worth visiting!
Firewall FAQ Frequently Asked Questions: A great overview of what a firewall is and what it can and can't do for you.
You need to check reviews on various firewalls out there?  Cool, here's a couple of places to look! 
Personal (Desktop) zdnet review, Fruit^Loop's review, SecurityPortal review, Firewall guide
Router Based zdnet review

Is it working?

Shields Up!  Want to check your firewall, go here and click on the "Shields Up!" banner.
LAN Help Questions about setting up mIRC to work with a LAN?


Yes, you still need virus protectionMcAffee pc-cillin Norton are a few.

And a good trojan scanner isn't a bad idea either. MOOSOFT (hey it's IRC, you can't be too protected!)