Network connection blocked problem

> I am able to get a dialog going with the computer that I
> want to connect to, but after the file transfer starts, I
> get a message saying that the Network connection is
> blocked. I have check the service provider on both ends
> (COX and SBC) and they aren't blocking. I have also
> checked the internet firewall option on under network
> connections. Both ends are turned off. One end is
> connected to cox cable and the other end is SBC DSL
> connection. The DSL connection is going through a router
> (Lisksys). This use to work, but it stopped all the
> sudden.
>
> Anyone have any idea why I am getting this? Any
> suggestions on how I should trouble shoot this?