Ports and modems

I would like to know what Windows 95 means by the error: “Unable to open port”. This error occurs when I attempt to test the modem on my user’s PC. I have com ports 1 thru 4 available in Device Manager. I have de-installed the modem and re-installed it, hoping Windows would assign a port not in conflict. This PC is a Compaq Armada 1700 laptop