SERIAL PORT BUSY, CAMERA WON’T CONNECT

Hav PIII, WIN98, pc bout 2 1/2 years old. WinModem went bad and replaced with serial exter modem, didn’t see USB. Had 2 dif ser cameras hooked to the two ser ports. Can’t get 1 of ports to work. Hav to use same port changing cables to use camera or modem. Says busy or not hooked up on other port. EASY FIX??? I’m pretty stupid so make it simple.