If you are useing your pc as a fax machine you might need to leave it on line 24/7. You never know when you are going to get a reply. Mute the sound or it will start to sing to you when the screen saver kicks in -- usualy about 20 min after you have gone to bed! I use 'Trio DataFax Voice' which came with my sound card and it does a good job of answering the phone and taking fax and phone messages. If you just want to send the odd fax it adds a printer to the list of installed printers and when you go to print your document you select it and it guides you through putting in the fax number, giving you the options of sending a coversheet etcwithout running the main prog.
Well, I use Supervoice which normally comes free with a dialup modem CD. It is only used to send faxes. To receive faxes without having to leave my PC on 24/7 I use Trinite at click here It costs £23 per year and all faxes received go into my email inbox.
I use Winfax Pro 10 for sending my faxes, (not cheap, but very good).
Like pj123, I use Trinite for receiving faxes; efficient and very reliable. The faxes arrive by e-mail in tiff format. You are given an exclusive 0870 number to use. The alternative, is leaving your PC on all the time.