So my roommates and i all purchased RS1. We wish to play the game together and are having difficulties with setting up a LAN in order to play COOP. The problem we're having is i run vista ( yes i know it sucks but it'll do for the moment) they both run XP. Is there anyway to set up a LAN with those two conflicting OSs?

The different OS shouldn't be a problem.

You will need a network switch, or a router with at least 4 ports.

Setup a network on each computer, and you should be able to find other computers on the network.

A router should auto assign IP addresses if you have that option selected on each computer.

If you use a switch, you will have to make sure each computer is using the exact same Workgroup name, and you will have to manually assign IP addresses.

Use IP addresses like:
192.168.0.121
192.168.0.122 .. etc.

The first 3 sets of numbers have to be the same on each computer.
The last set must be different to identify each computer.

You can change workgroup names by right clicking on "my computer"
select properties
Computer name
Then click the change button.