PC to Fax can provide a valuable Windows based fax tool.  It enables you to fax directly for their Internet-Enabled PC to any fax in the world.  Simply download the PC to Fax Client software, install it on the PC.  The user must have 'Allow PC to Fax' checked in the portal.

Download the software from below and install it on the PC.  Run through the setup and accept all of the defaults.

To run the PC Client once it is installed, simply open the document they want to fax, go to File->Print and select Internet Fax as the printer.  This will activate the GUI and the rest is self-explanatory.