You've got a virus/adware/spyware/hijack.
There are many different tools you could use.
Malwarebytes,
Combofix,
BoClean,
Adaware, even the
Microsoft Malicious Software Removal Tool are good examples of free software you can use to remove these things.
Sometimes, just looking at msconfig (Start > Run > msconfig) is enough for you to determine whether you've been hijacked.
Either way, download one of the free tools and get that junk off your system. The longer it stays, the greater the chance of secondary hacks/infections.
You could even do a System Restore and roll back a few days. Not the best solution, but it does work.
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