I'm having some similar problems, and have spent a lot of time troubleshooting based on forum discussions, but still can't get things to work properly. Here's where I am...
I have OS X, a Vista, the Keyspan high speed adapter, all drivers installed (on mac and pc). I have COM1 set-up in VPC and it's recognizing the keyspan adapter. I can get EasyGPS to recognize the Vista in COM1, and can transfer stuff that way. BUT, I can't get either the Vista software updater.exe nor MapSource to recognize it.
At times, the updater kind of begins (the screen on the vista changes), but it never gets beyond 0%. It autodetects the unit on COM1, so it SHOULD be working. I've checked and unchecked the highspeed setting. Humph. It only seems to work at all if I have it hooked to the USB port on the back, not the keyboard (thanks for the hint!).
For Mapsource, I can't get it to recognize the unit on any baud rate nor any COM port. Any port over 2 gives me an error, perhaps because VPC only lets you set up 2 ports.
Please help!
PS - I can use the updater on an actual windows machine, not that it probably makes a difference.