Assuming both machines have at least win98se installed I would have thought that the cheapest way would be to add a network card to both your Imac PC and principal PC, connect the two together by way of a crossed networked cable and using ICS (Internet Connection Sharing) as a software solution to share the Internet. The files etc can be shared via the standard file/sharing option in the network options of the control panel.
More effective might be a firewire connection between the two but a) I have never done that b) not even 100% sure it is possible to share broadband over firewire though I think I read it about that on a different thread recently and c) it may depend on your software level.
There have been other threads about this same question recently - may be worth doing a search...
hth, a little,
amofw