Make sure you buy an "ADSL wireless router" with a built in adsl modem (eg Belkin F5D7632 or Netgear DG834G). The easiest way would be to wire it to the PC with an ethernet cable (If PC has a network port, then its OK, if not buy a NIC ethernet adapter for £10 and install it (easy).
It is dead easy if it all works "just like that". If you have problems it's usually a case of posting here like you see to sort it out, usually resolved reasonably easily.
I know nothing about sharing files / printer between apple & XP. Internet will be fine.