If you don't already have a wireless adapter for the PC you can get a Linksys USB adapter, such as the
WUSB54G.
To recap, you need a router, eg Linksys WRT54G, and an adapter, eg WUSB54G. Some places sell router and apater bundles.
(Note - when setting the router/adapter up, it is best to have the PC connected to router via a LAN cable (one is supplied with WRT54G) - hopefully your PC has a LAN card!)