Ever since the 'real' serial comport (the hardwired connector on the rear of your PC) has been replaced with all types of 'virtual' alternatives like IR ports or Serial to USB devices, it has been a cause for all kind of trouble. And I've seen many of them, but the one I found this morning was new to me. Suddenly one of the programs I've written myself showed 18 comports to choose from. Now the program uses a dedicated function to enumerate the available ports, so obviously I thought it had something to do with a bug in my software. Also because the 'Hardware manager' of the PC only showed the two comports that are actually available. On the other hand I found that some other software that uses serial ports showed the same behaviour.
So I looked at the registry ( HKEY_LOCAL_MACHINE\HARDWARE\DEVICEMAP\SERIALCOMM) to find out which port were actually registered. And this showed all 18 as \Device\VComxx, where xx is the port number. So obviously something was creating virtual serial com ports. But what? I did not add any hardware to my PC that could cause this, nor did I install any serial com port related software or drivers.
So finally I located in the System Properties -> Device manager under 'System Devices' two suspicious entries: 'NCF Virtual serial bus enumerator' and 'VXCOM port'. The first turned out to be the 'Nokia Communication Framework', something I installed long ago in an attempt to communicate with my Nokia phone. After searching on the web for the second one, I found only a few entries, but it seems this one is related to the Windows Mobile 6 SDK, which I indeed installed last week. And when I disabled this driver, the problem was gone.
Apparently VXCOM is a virtual com driver used for the SmartPhone emulator. Why it reserves 18 ports at startup, even though I never used this emulator is a mystery to me...