You can't, really.
What you're best off doing is replacing the PC with a wireless router. Connect the router directly to the cable modem then any PCs near enough you can use ethernet cables to connect to the router.
PCs further away you can connect by adding a wireless card (internal - PCI - or external - USB) to the PC concerned which will then pick up the internet connection from the wireless router.
USB isn't really designed for networking so performance will be better too.
I forgot...