bootcamp drive wont format macbook air mid2013

this is the first time i post a question here

i recently bought a macbook air mid 2013 13 inches

i updated the mac to the latest Yosemite os release

i´ve got the need to install windows 7 so i bought a windows cd

i´ve read the manuals to how to install windows 7 on a mac via bootcamp

i´ve downloaded de compatibility software and partitioned my HD

after comming to the part where the disk partitions are showed i choose the bootcamp partition to be errased

i format it and click continue.

there a message shows up informing that windows is unable be installed on that partition

ive tryed several times and i allways get stuck in that part

i dont know what to do

i would appreciate anyone who can help