You can't really use the internet to send a fax directly. There are some fax gateway solutions you could use, where you would transmit the fax over the internet to a gateway fax server, which makes the call and transmits the fax.
You could also just get any analogue modem which supports the Class 1 Fax Modem specifications. This is basically anything faster than 14.4kbps, with very few exceptions. Simply connect the modem to the PC (or install if if you are an internal modem person) and hook up the phone line. You shouldn't have any problems using your ADSL phone line for this.
Some fax software (like WinFax Pro) will even allow you to set up one of your PCs as a fax gateway. Thus, one machine has the modem, and all your other machines can still use the service.
Willie Viljoen
Web Developer
Adaptive Web Development