Ok,Im not a vista user so im not all the familiar with it,but:
Click "Start" then "Control Panel" and double-click on the "Manage network connections" link.
Click the "Network" tab. You will see that Windows Vista is set to "Automatically assign an IP address."
Double Check it's Set to automatically assign and IP Adress,Also check the 'work offline' isnt on in internet explorer.
Lastly,if you are using internet explorer click on the tools button,and click on diagnose connection problems and it should tell you the problem there.