go to tools, internet options, and then check to see what your homepage is. if it's not what you set it and has been changed it's likely a program you downloaded and asked if you wanted them to change your page to theirs.
or, if you turned off your buttons (right clicking the top of the browser, put pointer over customize bar, and then 'turn off/on buttons') activate the home button and it'll pop up to the right of the tabs