Do you, perchance, have a router with DDOS etc protection?
Mine has "switches" in the software to allow or deny a whole raft of potential connections, from Kazaa through MSN Messenger, Skype, to pornsites and who knows what else. A power cut last year caused the router deep offence, and it turned all those ON.
I left most of them on, but turned the Skype-related one off.