click here for a MS article about ad hoc wireless networks and broadband connectivity. I think it is exactly what you want to do.
I think though that the real answer is to get a wireless router/modem that will connect all the computers with independant broadband access. Using the above, for example the main PC must be switched on all the time. Each computer will need to be wifi enabled using an internal wifi card or a USB Wireless adapter. I prefer the latter because you could just move the adapter between computers thus saving the expense of wireless connections on each one.
A lot of wireless modem/routers do have ethernet ports as well so you could just keep your current set up with cables. Here`s the one I use that will do all the above. Note that the "bundle" includes a USB adapter which would cost £30 bought seperately. It also has 4 ethernet ports. click here If you Collect from store you save £20. You don`t pay on line simply reserve it at your local store and collect within 2 working days.