you need to realise is that for faxing you don't use broadband.
From the transmission perpective "sending a fax" is actually making a phone call; it's just that the sounds going down the line are not speech but "whistles"
So if you're going to do it using a PC you will need an "ordinary" modem.
Assuming you have that then you can do it using WinXP. There is a "fax" feature. I can't remeber exactly how to access it, (I'm at work right now so can't check), but it might be via add/remove programmes, and look in the windows features options.
Once it is installed it's easy to do... You just get your document open, eg a "word" document, and "Print" it, choosing the Fax device as the printer.
I can't remember the exact sequence, but it's straightforward, and does work.
Note that I'm not sure what the FREE options are if you want to RECEIVE a fax!