I want to use my laptop (Toshiba L20/Windows XP,ISP:NTL)to send faxes by cable.
What type of modem do I need?
Do I need to buy or download any software?
Thanks in advance for any help.

You can either go with a PCMCIA Modem or an external Modem connected via USB or Serial Port (provided you notebook still has one) PCMCIA is more convinent for notebooks and better portable since it resides in the slot in your notebook. Anyway, all of the plain 56k modems should do the trick (its mostly says on the packaging fax compatibility). Nowadays I haven't run in a modem that won't fax yet. To the Software, you can use the programm that comes with XP >> go Start > Settings > Control Panel > Add or Remove Programs > Add / Remove Windows Components >> check the box Fax Services and Install it. It will install a Printer Driver as well which you use to print the documents you want to fax, after printing the Fax dialog should automatically open. To configure fax (for example for incoming faxes) you have to go into Accessories >> Communications.
Personally I don't like the Windows Program it never did would it should and then stopped working completely. I used the trial version of MightyFax for a while
MightyFax - Fax Software for Windows (http://www.rkssoftware.com/mightyfax/overview.html) (easy to use and did what it should for me) but most modems have a fax software included you might want to try that as well before you go out and buy something.