It's a pain but read here -
http://www.portforward.com/ , specifically here -
http://www.portforward.com/routers.htm . Basically what you're doing is telling your router to always give your pc the same ip address in your internal network, and then telling it to open the relevant port in the firewall for the IP address. Ish.
Take your time, step by step, write down/print what it tells you to write down, and you'll be there. I'd offer to help but I couldn't give any more help than what's on that site I sent to be honest, if you're (really) desperate you can try though! Someone said they knew a lot about routers and that on the "recommend me a router" thread, maybe ask them.
Good luck.