Yahoo has been getting worse ---- they were pretty bad before. If you install a yahoo program you need to go to the custom installation menu to uncheck the email, toolbar, etc features. It's not obvious. They don't neccesarily hurt performance, but they are, generally, unwanted clutter installed without permission. My son uses Yahoo Instant Messenger, and I am constantly going in and trying to turn off auto update and features that come with the updates. It has a life of its own.
Hijack This http://tomcoyote.com/hjt/
will let you easily change the registry keys that control search features that were taken over by Yahoo, plus show you other things you may not suspect are running when you start up. (but be careful what you change)