At the risk of being lynched by the Linux guys over here , a question about MSN messenger.

When I'm using MSN I can chat with friends, but I can't send files. Calling each other also doesn't work.
I can receive files my friend sends however.

I suspect this has everything to do with my router with built-in firewall. It's a UsRobotics USR8000 router. Now I know that I must allow more ports to be used or something like that, and I think I even found which ports on the microsoft site (here), but I don't know how to do this.
Can anybody help me?

Thanks!