It's the Bing addon from MS. They pushed it out as part of a series of updates a few months ago (though it can take a while for the update to be "Forced" onto your system). They did the same thing with Skype. They are trying to force people to start using it, because frankly...it's failing. It's not a virus, it's not adware (in a sense), but it's incredibly annoying.
You probably got prompted recently to "restart windows to finish applying updates", which is a dead giveaway that MS is forcing something down your throat, and then it suddenly appeared, right?
To get rid of it, do the following.
(XP, Vista, 7)
Go to Start -> Control Panel -> Programs and Features
Uninstall the Bing Toolber (and anything else that says Bing)
(Win 8, 8.1, 10)
Go to your Metro window, then start typing "control panel". It will pull up. Choose Programs and Features, and do the same thing.
AFTER it is uninstalled, follow any instructions for your specific browser on how to set your search engine back to whatever you want the default to be (most likely Google, I'd suspect).