A couple of things come to mind.
1) Disable one of your serial ports in CMOS setup, then manually assign it to the modem in the peripheral setup section of CMOS. This may be tricky, and I've never done it, but I think it would work.
2) Get an external modem and hook it up to your serial port.
Less work, more money.
3) If your computer has no serial ports, I guess my answers are totally worthless, and I am sorry to have wasted your time.