What do motherboard slots have to do with com1 and com 2????
The external com ports are usually located by the external mouse and
keyboard ports. If you're not using them, disable them in the BIOS.
Then you can install your internal modem in any of the available PCI slots.
(but don't use the PCI slot right next to the AGP slot).
The windows system will then pick a com port to use for the modem, usually
com1 or com 2 as they'll now be available.......
Greg