I assume that   you are trying to connect to a wireless router - you don't say.
Can you ping the router?
Try this: Go to network confuration and make sure that you have configured TCP/IP for DHCP.  That is to say 'obtain a network address automatically.
If this doesn't help or I have made incorrect assumptions please repost with more information.
BTW you may want to put this under a networking section rather than this one, which is for OS problems.  I only spotted it by chance.
                
 Terry
                
 September 2006