I had the same problem on my last ISP. I pretty sure there's a firewall problem. I solved it disabling Router firewall, NOT PC firewall. There was a checkbox on router config page called 'Enable firewall', but each router is diferent so... search for it and try it.
Edit: Now i remember... my problem was little diferent... i could see servers on my 2nd PC, but i couldn't connect to anyone
Edit: Now i remember... my problem was little diferent... i could see servers on my 2nd PC, but i couldn't connect to anyone