This repeats more detailed information found elsewhere. If your computer can't seem to find your serial port and you already know something about hardware resources (addresses like 3F8 and IRQs like 5) then try this: First, get into the BIOS (often called "setup") when the computer is powered on by pressing certain keys. To find out what keys to press, watch the screen as your PC starts up. If the words that flash by on the screen too fast to read, freeze them by holding down the "pause" and "shift" keys at the same time. Then when read, hit any key to resume (cease pausing) and hold down the key(s) required to enter the BIOS setup. You may have to try this again since there may be more than one screen which you can freeze with the "pause" key. Also, look for messages about the serial ports on these frozen screens.
Once in the BIOS menus, try to find menus dealing with the serial port. They could be shown in a menu dealing with Resources, Plug-and-Play, Peripherals, Ports, etc. Some old BIOSs setups (before 1995 ?) didn't deal with the serial ports. Make sure the ports you need are not disabled and note how they are configured (like 3F8 IRQ 4). You may need to change the configuration to prevent conflicts. There could be a shortage of IRQs if the BIOS has reserved some IRQs that it didn't need to reserve.
For serial ports to be found, either the kernel must have been compiled with serial support, or serial support must be provided by a module. To check this look in the file /boot/config-2.6... and search for SERIAL. =m means it's a module and you may check to see the modules that are being used by typing: lsmod.