Tallyho, have to tried the Windows program TelNet to check your port 30003 output? You can view the output from port 30003 using this program. Under WindowsXP this can be found on the \WINDOWS\system32 directory. Though as AirNav Support suggests above, reboot first and have nothing else running other than RadarBox and TelNet. Once you have started TelNet type in "open localhost 30003" at the prompt (without the quotes). After a while you should see messages like this...
MSG,3,0,0,400EFD,0,2009/02/20,11:31:58.109,2009/02/20,11:36:58.109,EZY9RA,32875,396,328,54.3889,-2.5191,-1024,6033,0,0,0,0 MSG,3,0,0,4006B9,0,2009/02/20,11:31:58.109,2009/02/20,11:36:58.109,TCX652K,12900,333,264,53.6832,-2.3085,2048,1160,0,0,0,0 MSG,3,0,0,400A14,0,2009/02/20,11:31:58.109,2009/02/20,11:36:58.109,EZY10XH,39000,391,324,52.9354,-1.4900,2240,5430,0,0,0,0
that will tell you at least that RadarBox is outputing something to port 30003.