http://www.foundstone.com/us/resourc...desc/fport.htm
Use FPort to determine which executable has bound itself to 3389. Although it would be an unusual situation where a third party application takes that port, as its pretty much the defacto standard for RDP.
Another thing you can try quickly to isolate whether its a router issue with port forwarding or a machine issue is to run "mstsc -v localhost" from the run menu, and see if you can RDP to yourself - if you can, it's not likely the machine, its probably the port forwarding. Has your static IP address on the box changed?