Networking Issue?

Alright, my friend is over here tonight and we were trying to network our computers. He's plugged into my router and he can browse the interweb just fine, but programs like AIM, MSN, Trillian, don't connect. We also can't network our computers, we can't see each other, etc. Can someone attempt to help me out with this? Maybe a list of things I can try? Theres no firewalls, my router doesnt have a firewall, so I dont know. I'd appriciate some help if anyone has any ideas.

Does the router have a built-in switch?
Do you have DHCP enabled?
Are you in the same workgroup?

>>2
No.
Yes.
Yes.

:\

does you both having teh intarweb?
does you haivng two ip addr from isp?

You likely have some cheap/old router that doesn't have built in support for automatically doing NAT for AIM protocol. It doesn't have a built in switch? Are you saying it is a hub then? Can you ping each other? Do you both have the same subnet mask?