Wired:
Hook the PS2's network adapter to one of the router's LAN ports, using a straight-through ethernet cable.
Wireless:
Get a wireless bridge (e.g. Linksys WET11 or WET54G, or Linksys wireless gaming adapter). Set it up via the PC, then connect it to the PS2's network adapter.
I don't have a PS2, so there's probably a bit more to it than that! But I think that's the general gist of it. You'd also need to configure the PS2's network adapter - I believe it either comes with a CD for this or you have to order one.