If you type
http://216.239.57.99/ you should see googles website.
Ok, what you need to do, is Release and renew your IP address.
Goto Start, hit run and type in ipconfig /release and hit enter
then do it again, but type ipconfig /renew
There is also a chance that comcast doesnt have the mac address of the network card registered in there database, which may be causing you head aches.
Restart Modem
Restart PC
Release and Reniew IP address
If all above fails , call comcast @ 888-262-6300