Strange problem. Does the computer that you are using have a global non-translated IP address? It might be possable that your network is setup such that your router which is doing NAT is translating incoming ports to different internal hosts. It is more probable that device doing your translation is a piece of crap and needs to be drop kicked.

If you are using a cheapo POS home cable modem router, see if you can borrow one from someone else, drop it in there using all defaults and see if you have the same problem.