If it is an add in pci/isa card usually it will have an irq setting or jumper on the card, there is a trick to set legacy pci/irq's in the bios, most internal modems work well on com4 irq is usually choosen based on what is NOT in use by other devices, most modems will work with default hayes compatible drivers but if it is 56k or x80 v90 etc you will need the correct drivers for that modem for your OS once you have setup your irq in the bios you go into windows device manager and configure the modem manually to use that com port and IRQ, a quick "diagnostics" check via the modems tool in control panel will tell you if the modem will initialise with the driver you have installed. From there you setup your dialup connection to use that modem as default. you may want to have a look at this artical from microsoft http://support.microsoft.com/kb/152064/en-us