PCs have from one to four COM (serial) ports. These ports are many times associated to serial sockets on the back of the PC. COM ports are used to connect serial devices, such as a mouse, modems, PCMCIA sockets, and other computers to the PC. Each PC usually has two serial sockets on the back which are associated with COM ports. Devices such as modem and Infrared Drive may be installed internally, but they still make use of a COM port, even though they are not connected to a serial socket on the outside of the PC. In these cases, the PC redirects the COM port via the PC slot to the card. If the device uses a COM port which is normally associated with one of the sockets on the back of the PC, the port is taken away from the socket and given to the device. The implications of this will be clear in a moment.

Each COM port gets the attention of the computer's processor by means of an Interrupt Request, called an IRQ. Think of these as "channels" by which the COM ports tell the processor that there is data waiting. There are only 8 IRQ's available to standard serial devices, and several are already dedicated internally. Because of this, COM ports 1 and 3 (if installed) share IRQ4, and COM ports 2 and 4 (if installed) share IRQ3.  This usually means that if there is a serial device, such as the mouse, using COM1, and another using COM3, such as a modem, the two devices will experience an "IRQ conflict" which is likely to disable both. These conflicts must be resolved before the devices can be used successfully.

PARDON THE INTERRUPTION

An interrupt conflict can be a true pain. To help you avoid such conflicts as much as possible
(and solve them when you can't) here's a list of interrupt assignments.

           0 Timer
           1 Keyboard
           2 Cascade (for interrupts above 8)
           3 COM2, COM4
           4 COM1, COM3
           5 LPT2
           6 Floppy Controller
           7 LPT1
           8 Clock
           9 Sometimes used for ISA in a PCI computer
           12 PS2 Mouse Port
           13 Math Coprocessor
           14 IDE Controller

           Note that interrupts 10 and 11 are generally not used.