this is easier than you might think!
Are you in the UK? Is it ADSL (Telephone) or is it DSL (Cable broadband)?
Assuming that you have a telephone broadband system.
Buy an ADSL Modem Router - something like this
http://www.nickknows.com/description...67&category=88 (I just found this by googling, I've no connection with the company!) - The Netgear DG634 is nice!
Buy some cable and some network cards if there aren't any in the PC's already!
You now have everything you need!
This box becomes your gateway, and you can add an ethernet based Wireless Access Point to it at a latter date to give you a wireless connection (or buy the wireless version anyway!)
To set it up its just a case of putting it together, network cards in all the PC's, cable it up and switch on the router. This will automatically make contact with the broadband and start broadcasting to your lan.
You need to set up the network cards in the PC's so that they know where the gateway is (possibly 192.168.0.1) and set it up for DHCP. That's netconfig on the LInux boxes, Control Panel - Network connections on XP
Once everything is switched on, it should start talking to each other and you can just do your internet thing!
BTW, this might sound very easy, but RTM just to be safe!
Hope this helps!
F0ul_Oli