I just helped another person with what is sounding like a common issue, the only difference is they were reporting that the page would come up with page cannot be displayed and their home page had changed to a websearch page. It seemed to me that they had malware that was hijacking their preferences so I directed them to download the coolwebsearch removal tool from trend micro. I'll give you that link too, it's totally worth a shot because it's small and runs quick and it's better to get that possible problem out of the way. Download and run the program below, choose to fix right away as opposed to scan and if it tells you to reboot do it right away.
If it doesn't detect anything we're going to have to take the troubleshooting up a notch.
So if that doesn't fix it we need to see if another malware program is redirecting you so you cannot get to yahoo. Go to c:\windows\system32\drivers\etc and open the hosts file. A standard generic hosts file usually only contains this:
# Copyright (c) 1993-1999 Microsoft Corp.
# This is a sample HOSTS file used by Microsoft TCP/IP for Windows.
# This file contains the mappings of IP addresses to host names. Each
# entry should be kept on an individual line. The IP address should
# be placed in the first column followed by the corresponding host name.
# The IP address and the host name should be separated by at least one
# Additionally, comments (such as these) may be inserted on individual
# lines or following the machine name denoted by a '#' symbol.
# For example:
# 188.8.131.52 rhino.acme.com # source server
# 184.108.40.206 x.acme.com # x client host
There may be a few lines below the localhost entry but if you see any references to www.yahoo.com
then we know you have some really bad malware and need to run a good spyware removal tool. For that I recommend Spybot Search and Destroy and Ad-Aware. I don't really trust anything else really.
If that doesn't work the last thing I can suggest is trying to reach the website by IP address. Go to start>run>type cmd then hit enter. In the command window type ping www.yahoo.com
and it should return the IP address. Type that into the address bar in your web browser and hit enter. If it gets to yahoo then you have a DNS issue. Let me know if that is what happens and I'll send you the instructions to change the DNS address on your network card.