Some applications (especially mapping programs, it seems) choke on virtual serial ports. I have had the same problem that you've reported with MapPoint and ComFoolery)
The alternatives (that I've found) seem to be:
1 - Other virtual port software:
I am using ELTIMA (
http://www.eltima.com) Serial Splitter right now. But, don't know what I'll do when the trial runs out, as the $99 price tag is too hefty for me.
Some GPS applications VisualGPSXP (
http://www.visualgps.net/VisualGPSXP/) for one, also can split the GPS signal. VisualGPSXP broadcasts on a TCP port just like ComFoolery, so it may not work well either.
2 - Connect NetStumbler directly to a com port and use a Script from the Scripts forum to relay information to MapPoint.
3 - Hardware solution through Y splitter cable and possible use of additional com port through USB / PCMCIA / other hardware.
4 - .... ????
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