Hi there. First time posting here at laptopgpsworld.com.
I have a Garmin GPSmap76 connected to a 1001HA EeePC (WinXP sp3), using a Serial-9-pin -> USB adapter (HL USB-RS232 adapter (blue package, greenish plugs, silver cable)), and adapter driver XCS-232-340 installed to PC via Control Panel > Add Hardware. There are no doubt more suitable adapters, but this one from China was cheap, and it works for me.
Hope this is some help.