Networks pc-cabled>pc-wireless>router problems!

ive just got myself a router and i'm trying to set it up so that a computer downstairs is connected (by wires) to it, a second computer upstairs is connected wirelessly to it and a third, also upstairs, is connected to the second with cables which is then bridged with the wireless connection (using xp). currently i have two pretty major problems-
1 - whenever the 3rd computer attempts to get an ip address from the router dhcp server it causes a ip conflict error in windows and the 2nd computers ip changes to that of the 3rd!
2 - neither of the upstairs computers can access any external ip addresses, but the wired one downstairs can.

im pretty lame when it comes to networking so any help most appreciated!

cheers

chris