Open cpn & garmin

[Tazzie]

Can anyone help me, I have downloaded Open cpn to my PC (Win 7). I have a garmin 152gps in my boat which is interface to a seatalk unit. In the communications menu of the GPS I have set as NMEA To NMEA. I have a serial/usb cable running from seatalk unit to PC. I have had the system navigating a couple of times but am unsure what I did, but for the most part it will not talk to GPS. The previous owners of the boat had the same setup ,but running off a much older PC (no usb ports, nine pin plug only ) and it ran well . Once I upgraded to modern PC the trouble started. I have a programme called GPS info downloaded onto PC and this is receiving status information from Garmin GPS so this tells me that there is some sort of a connection between GPS and PC, any input would be appreciated, Tazzie