Since you already have MapSource on your computer you can download and install Garmin's nRoute program. It looks just like MapSource but adds full navigation capabilities, complete with routing, voice instructions and everything.
The only issue with nroute is that it only works with Garmin's proprietary protocol. So to use it with your generic receiver, which outputs the NMEA protocol, you need to use the Franson GPSGate shareware program. It reads the NMEA data stream from your generic receiver and outputs the Garmin proprietary protocol on a different (virtual) COM port. You have a trial period for GPSGate so you can download it and nroute and try them out to see if this is a solution you could live with.
There is more info on nroute and help from other users here:
http://www.laptopgpsworld.com/garmin-mobile-pc/.
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