Garmin used to have a program available on their website called nRoute
that allowed you to use a Garmin GPS with a laptop. You needed to first install Garmin's MapSource software. I doubt nRoute would work with any of Garmin's currently shipping GPS units but I have an older Garmin GPS, a StreetPilot 7200, and it works great with the nRoute software on a laptop.
There are plenty of other ways to use a GPS receiver with a laptop. Check out Delorme's Street Atlas
software which can be purchased with or without a GPS receiver. Microsoft has Streets & Trips
. This software may not work with your particular GPS but there are plenty of GPS receivers out there that can be used on a laptop with this software.