red said...
Device manager shows USB to Serial Adapter as working and I've gone an played with all the COM port settings seeing if that would make a difference but no joy. All the different software packages just do "see" the GPS
What type of Garmin is it? There is probably a default setting for the COM port settings depending on the model you have.
What settings did you use for the com port? Common bit rates are 9600, 19200, and possibly 4800. 4800 seems common for NMEA output, although I suspect 9600 would be the best guess. Your other settings should probably be 8 data bits, no parity bit, and 1 stop bit. If that makes no sense to you, let me know what you see in the COM port settings section.
If there is a section for flowcontrol, set it as none, as it shouldn't matter for testing, and probably wont matter anyway.
Do you have hyperterm (a serial terminal emulator) installed? You can use this to test COM ports by sending and receiving data, although I don't know if it comes with Vista.