I installed Mappoint 2004 and connected my usb Earthmate with the usb-to-serial drivers supplied by DeLorme. MP detected the earthmate but then I got the error saying the data was being received but it wasn't NMEA compliant. Then I installed AGPS, the COM add-in for MP. When I set up AGPS to recognize the earthmate, it immediately locked on and everything was working fine. Then I turned off AGPS and tried to track with just MP. It also started tracking with the earthmate.
I'll keep using AGPS though because it has autorouting and voice prompts.