Try physically removing the modem from the computer. If you have a network card installed try removing that also. Turn off the computer, reboot. Now install just the modem. Power up. and let windows try to recognize the installation and find/install drivers for it. If it does not find the drivers you will have to download the right ones or off CD that came with the modem. After reinstall the network card NIC if needed.
Also, in some older PC Bios configs you had to enable modem IRQ ports. You might want to check your BIOS also just to make sure.