From skimming some of the common problems in these threads, it could be that the ISP has something shut down inbetween you and the real world. I've had major issues getting Verizon to open ports so I can video conference. A few of my users can't access our corporate mail server from a Verizon connection either. Many conversations with Verizon support have all yielded this gem of helpful wisdom: "The Verizon terms of service agreement (TOS) stipulate that you will not run a server or server-like computer from inside our network." It's like some of the crap ISPs up here in Alaska... they'll let you download all you want at decent speeds, but try to put anything UPstream, and you're SOL....
Drag your PC over to a different brand of internet connection and see if things change any...
-Case