Sounds like something is trying to DNS poison Yahoo.
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The first thing I would check is the primary DNS server your machine is using (if this is a Windows PC, check network settings), router DNS settings, try a different DNS server, clear your browser cache, history, auto-complete history, etc.
If you are doing this from your own server (using X, gnome, KDE, etc), then check what contents you have in /etc/resolv.conf (RH servers). I have seen instances where people have had DNS resolution problems due to mis-settings on their server.