How to delete my most visited websites?

ok my idiot nephew looked at on my computer now everytime i open a new tab instead of listing just my usual top ten it also lists the site as part of that.....my dad goes thu my computer all the time and i dont think hel belive me if i tell lhim that my nephew was on that site and not me please tell me how to get rid of it....ps in case u didnt no everytime you open a tab in my computer instead of going to google it shows all my most recent websites or top ten i also have windows vista pleeeee heelllpp!!!