As you don't want to have to have both PCs on then you will need a wireless router and a wireless adapter (USB dongle for e.g.) Your modem MUST have an ethernet port to connect to the router. If not then you will need to buy a modem-router (all in the one unit) and keep the original as a spare or sell it to recoup the cost. Modem routers are not much more expensive than modems on there own. The big names (Linksys/Belkin/Netgear etc) are now selling router and adapter packages now for around £40.