Antivirus is a touchy thing, people debate endlessly over it. Choose one from reviews online and go with it, as there are sites that rank them periodically). None are infallible, and you can't layer them.
Add on for defense Ad-Aware and Spybot Search and Destroy for malware.
Get the Sysinternals Suite for troubleshooting. Invaluable.
Other security depends on your setup. Is the machine virtual? That can help for monitoring as anything that affects the computer is a little more visible by a "virtual" third party, to see network traffic and behavior. Is the machine at home, or in a business? Make sure it's behind a NAT firewall or a hardware firewall to help protect it. Software firewalls are okay, but in many cases the number one defense comes from just blocking off all non-essential for external access ports at the Internet ingress point...your cable/DSL router or NAT router.
Any other software depends on what you're going to do with the system. Web server? Database? File server?... Servers, generally, should have only what's needed to fulfill their role on them. That alone helps with security, as the more crap you later on it the more surface there is for attacking the computer (and more bugs and unforeseen behaviors can emerge...)