Before I go through the hassle of proving this to myself, I'd like to know a couple of things about your particular installation.
1. Did you upgrade an XP machine that had a working copy of WinSPM on it?
2. Is this a single experience with one machine, or has this been proven on multiple machines?
3. What COM port assignment are you using?
While Vista is a new O/S, we are still talking about emulation software talking to a serial port; nothing complicated. Windows XP has the ability to address a greater number communiation ports (COM10 and up) by using different Base I/O Addresses and Interrupts. That's great, since all the new stuff like printers uses serial ports. But WinSPM won't work in these higher ranges, even on XP.
If this is an upgraded unit, you may have old and unused software/drivers with "hooks" on one of the preferred ports (COM1 or COM2). One sure sign of this is an inability to assign devices (like a modem) to one of these lower port numbers.
There is a nice post at
describing a technique for freeing up the low-numbered COM ports by killing a "nonpresent device". In this particular post, a previous installation of BlueTooth software had created a "nonpresent device", which unnecessarily occupied the needed serial port.