It's an add-on that's been installed at some point on your computer.
In Internet Explorer go to Tools... Manage Add-ons... and disable or remove one called YONTOO
If you use firefox, just click on the Firefox dropdown (top left) and select Addons; Extensions;
You will see Yontoo in the list - just disable it and restart Firefox
That's what google tells me at least... :)
Good Luck.
Tyler