One of the few utilities i've found worthwhile is this:
Yasu :
http://www.jimmitchelldesigns.com/software.html
You should run it every once in a while, like each month to clean cache's and keep everything clean, best of all, it's donation-ware, meaning payment is optional.
You don't need an antivirus (8 years without any virus for the platform) but if you want to scan all the mails the "other people" send you and be able to clean them to avoid forwarding virus, you might get Virex, it's included with .Mac accounts.
And, if you surf a lot, it could be important to turn on the included firewall (find it in System Preferences - Sharing, 2nd tab)
But, mind you, these tools are mostly unneeded, my parents have kept using a system for over two years without any problem/performance hit.
And, by the way, Welcome to the bright side, i hope you'll find, as most of us have, that computing is really about making your mind free, not your hands busy ;-)