[Rickertsen2]

Windows allows access to serial port by only one program at a time. If one program already has it open, additional connection attempts will fail. To my knowledge, there is nothing you can do about this other than have one of the programs close the port before the other tries to open it.

As a side note, if your program crashes with the serial port open, there is no way to close it and make it available again other than restarting the computer. This is true in windows 2000. I assume it is true in other version as well. I share this because it caused me a day of frusteration thinking there was something wrong with my program.