Power cycle the router, might even want to hit the reset button, and try again.
Can you get internet if you plug your PC directly into your cable or DSL modem? You might have to power cycle your modem so it will read the mac address of the NIC in your PC. It will be looking for your router. If you get internet then the problem is with the router. If you don't then their is a problem with your PC. Your NIC is set to automatically obtain an IP right? DHCP is enabled in the router?
You can also try this.
Open a command prompt, in XP start => run => cmd
Then run ipconfig/release followed by ipconfig/renew.
Then run ipconfig/all and see what you get. Post it here if you want so I can have a look at it.