How do you stop adverts appearing on right side of page when viewing Yahoo mail in Vista Windows ?

I have an advert that keeps appearing at the right hand side of my Yahoo mail window, is there an option in Vista Windows or Yahoo mail to personalise this page to stop this advert, it shrinks the size of the area to read mail and you have to click a on to remove it but when you open or close a mail item it reappears again. How do you stop it???