I'm running Windows XP Pro and the computer has two seperate gigabit NIC's. I would like to assign one NIC strictly for LAN use and the other for Internet. Any ideas how to set this up? Or is special software required to do this?

I'm not exactly sure what your trying to do but here are all the situations you could be going for

Situation 1
no internet forwarding to lan

Sugestions
cheap
buy a hub to connect to the lan
manually assign ips

slightly more expensive but easier
plug lan into router ignoring wan port
router will suplly ips

Situation 2
Intenet connection sharing
just run the network setup wizard and enable ICS
your computer will assign IPs to the rest of your network through a switch, hub, or crossover cable
Situation 3
Network Bridge
open up network connections highlight the two ethernet cards right click and select bridge
you have to manually assign ips to the lan side but all computer will have access to your modem or router controlling the internet not just the internet signal. Its also useful in different situations for bridging a wireless connection and still giving complete access to the whole lan.

example I have a computer receive wireless connection from router then through hub to xbox or another pc both the pc and the xbox have access to shared files all across the network and the pc can configure the router.

Things to remember you cannot connect connect two computer without a the switch on a router,a normal switch,a hub, or a crossover cable

I'm trying to seperate internet and lan traffic for added security.

My LAN consists of 5 computers with dual gigabit connections; they are connected to a switch that is connected to a router.

Well then you have two options

option 1
Directly connect the internet wire that was in the wan port of the router to lan card 1
Directly connect lan card 2 to switch on router

cons
if your router doesn't have a gigibit switch it's going to be slower
your exposing your main computer straight to the internet definitely less safe for that computer without a hardware firewall. If that computer gets compromised then all this was useless. You could try it through the wan port then set up a vpn to gain access to your lan (still volunerable if vpn password is compromised from lack of hardware firewall).
pros
easy although setting up a vpn can be daunting

option 2
connect lan of router to lan card 1
Connect lan card 2 to switch
manually assign ips

pros slightly more secure than option 1 without vpn(still vulnerable if first computer compromised since it will have lan access)
cons little harder set up

option 3
connect lan of router to lan card 1
buy second router or set up an old computer as a router and plug into lan2 into wan port then connect the routers switch to your switch
setup a virtual private network for first computer to network with lan
openvpn is free and secure

pro
this is the only real option that provides actual security for your lan since both computers will have hardware firewalls and the lan is only compromised if your vpn password is.
cons
A bitch of a set up
have buy or diy a router

personnally, I think this is a little excessive as long as you have antivirus up to date and keep adware under control you'll be fine with a normal router set up.