I presume you are trying to connect to the Internet via a wireless network, is this correct?
What kind of wireless router are you using? Is its firmware up to date?
If you go to System Preferences > Network > AirPort > click on the Advanced button and then go to the TCP/IP tab, what does it say next to Configure IPv4? Do you get an IP address? If so, what is it?
Let us know, we will keep working with you.
Best of luck.