I think in some Vista version there is a built in Windows Fax already. You need a fax modem and a phone line to make it work. Go to Start - All Programs - Windows Fax and Scan and it's pretty much user friendly from there.
If you don't have a fax modem then you can also opt for an online fax service instead of buying a fax modem. This way your fax will go through the internet thus no need for a phone line either. I use online fax service from Onesuite and it works.