Make sure you know the IRQ and COM settings on the modem (if it's a hardware modem).
Then configure the modem IRQ in BIOS to the same.
I've had trouble using COM 1 and 2 on the Abit board. It's better to use COM 4 or 3 since they're not used by the serial ports.
Get Windows to detect the new COM port (add new hardware)
Then install the modem drivers (since Windows will most likely not detect the modem, even with modem search).
Make sure that you set the modem on the same IRQ under windows as is set on the BIOS/modem jumpers.
And then make sure that there are no IRQ conflicts. I used to keep getting IRQ conflicts with the ACPI and couldn't fix it.
If all fails, get a PCI hardware modem

Much easier.