I have a linksys 4 port router and using it to shared my cable internet connection betwen my two computers...

the problem here is i want to upload a file to my friend over msn and mirc but it never works, it said that my firewall stop it. well, i know how to forward my port to work with mIRC but each time i do that, it will stop my other computer from getting on the internet, but each time i undo my port forwarding, it work as normal again... why? each time i forward my port, it does let me upload on mIRC but it stop my internet connection on the other PC...

can anyone tell me how to make it to works?

here is my setup:

http://home.attbi.com/~hien.danh/lelele.PNG