Normally, when you plug any USB device (in your case, the serial-USB converter) into a USB port, Windows will ask for a driver. Did this occur? If not, it is either already installed or disabled. You can check in Device Manager under Ports (COM & LPT) to see if it is there. It may be similar to the attached screen shot. This will also tell you which COM port to use.
To access Device Manager in XP, Start-Control Panel-System. On the Hardware tab select Device Manager. Or right click on 'My Computer',choose Properties and the Hardware tab. In Vista, type Device Manager in the search area on the Start menu.
Unless your version of MapSource is really old, it doesn't support a GPS. I don't think MapSource has supported a GPS since version 6.5; the latest version is 6.15.6. Do you mean Garmin Mobile PC or Garmin nRoute? If it is Mobile PC, there have been other posts about it not supporting the Etrex. If it is an old version of Mapsource or nRoute, point that program to the COM port listed in Device Manager.
Terry (61)