you have many options....
one could be to:
1. Install 10/100 network cards in each pc.
2. Purchase a mini 4 or 5 port switch.
3. Share your broadband connection on your pc to which it is connected.
4. Cable all your pc's to the switch.
5. Follow the Windows ICS (Internet Connection Sharing) instructions you can find all over the web - just do a search on google.
If you don't want cables everywhere, then you will have to look at a wireless solution. Then you need to consider replacing your existing broadband modem with a combined switch/router/modem device.
Hope that helps a little.