Which version of Windows are you using? XP has the ability to send/receive fax messages built in; with Vista, only the "business" versions include the feature.
In XP, open the "Printers & Faxes" folder and click the link at the left to "Set up faxing". The PC does, of course, need to have a "dialup" faxmodem installed and any V90/V92 modem should be suitable. An ADSL modem is *NOT SUITABLE* as it is an internet-only device and fax is transmitted using a conventional phone call.