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Think it's some sort of virus or malware. At my work all of our computers there have been redirecting to that shit but from google but it doesn't happen to me at home or on any other computers.
Probably a problem with your hosts file. Try downloading McAfee Stinger (google it) and run that, I think that will fix your hosts file. Also use Spybot Search and Destory to look for spyware
Did u install any extensions/plugins that may inject codes to redirect yahoo search?
Also use adw cleaner, as it removes traces of viruses, malewares etc...