Usually, automatic DHCP will giv eyou an address related to your Default Gateway but as you don't have one, you need to force the system to turn it on prpoerly. Go into Control Panel>Network Connecitons (assuming XP) or Network and Sharing Centre is Vista or Windows 7. You need to be wired to the router when you do this.
Highlight the Local Area Connection, right click and click on Delete. Do the same to the Wireless connectoin then restart the computer. If that didn't do the trick, pelase post back - you need to set the addresses manually - probably and hopefully only once.