I formatted my laptop from factory default windows 7 home prem. to windows 8 , before I formatted my laptop to windows 8 I delete all the partition that is from factory default to clean install windows 8 now I would like to go back to windows 7 and use my key that is under my laptop. but when I try to install windows 7 again and enter the product key it wont accept it , what could be the problem? and how can fix it? thanks in advance!!!