sounds like the router isnt giving out an I.P address to the computer/s......have you changed the default i.p address of the router...(192.168.0.1 or 192.168.1.1) ?? your router will have come with a manual telling you the i.p address if your not too sure...if you havnt changed it try goin into
Start>Control Panel>Network Connections>Right Click Local area connection>Properties>Double click TCP/IP>Use the following I.P address....
then give it an i.p address of 192.168.0.110 or 192.168.1.110 depending if you have changed it or not
let me know how u go