Invalid Communication Port or Busy

I recently installed a Motorola 56k v90 pci modem on a system with winxp 256ram and amd 1.6mhz. The modem is xp compatible. I installed this modem a week ago and everything worked fine. I was using aol and it’s email program without any problem.During startup,it displays “found new hardware and is locating software”, but I have not installed any new hardware since this modem.
After progressing to the aol sign on screen and clicking sign on button this message appears “Ivalid CommunicationPort or Busy”. Nothing has been changed since the successful modem installation. Why can’t I get passed the above communication error message? Any help will be deeply appreciated. Thanks.
Len Piko