The software for sending faxes from Windows via an old-stle faxmodem is
built-in to Windows 2000 and XP, although you may have tio install it using
Control Panel|Add or remove programs|Add//remove Windows Components, at
least in Windows 2000. Once installed, there are various possible ways to
send a fax, including printing from the application to the "Fax printer",
whose driver asks for the necessary faxnumber etc. The people in the Outlook
newsgroups/communities probably know more about that, and the newer Internet
fax services.

If your customer is trying to Mailmerge to fax from Word, see

http://tips.pjmsn.me.uk/t0001.htm

If your customer wants to send via the Internet, there are fax services
which you have to sign up for. They may work in a variety of ways (e.g., you
might have to e-mail the fax, or print to a "virtual fax" device). Looking
for "fax services" on Google seems a reasonable place to start.

Peter Jamieson