· Your computer can have up to four COM ports, but your computer restricts how you may use them. The general rule is to avoid using COM1 and COM3 at the same time, or COM2 and COM4 at the same time. For example, if you have a serial mouse using COM1, your modem may use either COM2 or COM4, but not COM3.
· Similarly, if you have a serial mouse using COM2, your modem may use either COM1 or COM3, but not COM4. (The technical reason for this is that both your mouse and your modem use "interrupts," and your computer usually provides two interrupt request lines (IRQ's) for up to four COM ports.)
See Checking Your COM Ports with MSD.