help with setup / configure of a LAN

I currently run 3 PC’s all seperately though 1 common 2-wire gateway router. 1PC is WinXP-sp2, the other 2 are Win98. The XP machine is connected by cat-5 cable, while the other 2 are wireless. I want to set up a LAN between the 3 computers, but I also only want the XP machine to have access to the internet (I want to block all internet access to the other 2 computers). Can this be done? How, (both physical connections & software setups)?