|Well there is not that many applications that would want to use the serial port. What devices, if any, do you have plugged into the serial port? I take it we are talking about RS232 serial port and not a USB port.|
It would have to be some kind of communications application, like Internet Explore if you were on dial up. The Internet dialer closes the serial port when you log off. Or a serial mouse.
If you are getting unexpected use of the serial port I would check for the presence of a rouge dialler. Do you have a good Anti Virus installed?
Then there are some badly written applications that do not close the serial port when it has finished with it. Is it your own application that is keeping the serial port open?