Search for BU-353-W. I've used this antenna with Fugawi with great success on Windows XP, and there is a USB to serial driver for Windows 7 at the website where you find the antenna. The antenna works with everything I've tried (Streets and Trips, Chart Navigator Pro, Google Earth Pro, Fugawi, etc.)
I've also used my Magellan Explorist XL as the antenna for all those applications. If you want to be able to use a standalone hand held GPS receiver that doubles as an antenna for your notebook when you need it, then you should replace your Garmin.