There is a standard serial protocol that is used. I imagine that if the cable connects the GPS unit to the PC via the serial port, a moving map type program on the PC would be able to retrieve position data from the GPS unit.
Take a look at some of the GPS-aware software out there to see if they mention support for your particular GPS unit. If you don't see anything, go to groups.google.com and do a search for the name of your GPS unit and "moving map" or something similar.