Have you previously interfaced the PC successfully with that GPSr?
On my Legend C, I had to uninstall the drivers that Windows found, and reinstalled a driver I found on Garmin's site.
Also, I screwed up initially by connecting the GPSr before loading the drivers.
So I had to uninstall the driver, shutdown, restart, reinstall driver, shutdown, restart, and then the PC recognized my Legend C by name, when previously it thought it was an "external storage device".
I also found that my PC likes me to connect the GPSr while OFF, and then turn on the GPSr once I've opened Mapsource.
Otherwise it doesn't always like to interface.
(Of course, I've had tons of issues with my PC lately, it doesn't always recognize itself, I think it had digital dementia!)
Good luck!
-Kris