Your problem may be caused by a browser redirect virus. You need to remove it so that you can use Google or Yahoo normally. Here are guides to remove browser redirect virus: http://guides.uufix.com/category/browser-settings

Sounds like you are experiencing "Browser Hijacking". This is caused by a malware that can be tricky to get rid of at times.

Download and install the free program at the following link. It is really good at removing many type of malware that normal virus scanners miss.

Http://malwarebytes.org