I'm assuming it's an external cable modem? Easy then, just buy yourself a cable/DSL router (like Linksys EtherFast router, or Netgear RT314, etc... there's tons of them out there) for around $150-$200. That's all you really need (and a NIC in each PC, and the cat5 cable to go between them of course).
The cable modem plugs into the WAN connection on the router, and the router has 4 ports (a built in switch/hub, some have more ports) on it's LAN connection to which you can plug your PCs into. They're very easy to set up too...
If you have an internal modem it's a bit more difficult. You have to install a NIC in that PC alongside the cable modem, run some software like WinGate (or even ICS that's built into Win98SE might do the trick) and then it's a matter of using a simple hub/switch to connect the PCs together.