You've been hijacked
Apart from manually setting the default home page and search settings, download and run these tools one by one
Hitman Pro
Downloads - SurfRight
Smitfraudfix
How to use SmitFraudFix - Geeks to Go Forums
Combofix
A guide and tutorial on using ComboFix
Malwarebyte Antimalware
Malwarebytes Anti-Malware - Download.com
Then install BroswerProtect
Browser Hijack Protection for free - BrowserProtect.org
These are not alternatives - you need to run them all, consecutively
Smitfraudfix and Combofix should be run in safe mode, the others are OK in normal mode. Essentially, whatever they find, delete
Which version of windows are you running, and what security software?