You should try running the tracert command from the command prompt. If it behaves similar to PingPlotter, then you know it's not a problem isolated to PingPlotter, but it's something to do with your machine. Keep looking for software that might be acting as a firewall.

If tracert is different than PingPlotter, then you should check packet type setup in PingPlotter. I suspect, though, that it's not a packet configuration problem.

Good luck,
Pete