I think what Electro says is logical, however, sometimes it's a simple DNS problem. If you were already using another ISP or even another product of the same ISP, your DNS Server may already be set to search for Domain Names on the original server (it's automatic). What you may need to do first is to delete the old ISP service off your computer. Even if you've disconnected, if it's still software installed on your computer, that's where it searches for Yahoo. If that is happening as soon as you do what I suggest you'll find the web sites will be contactable, because Ie or another browser will search for the nearest DNS Server and find one within your new connectivity.
Keyhole Kate, February 2012