installing windows 7 on Dell 7537

hi i want  to install actually do a dual boot to my dell 7537 of windows 8.1 (pre installed) and windows 7 .

I have downloaded windows 7 ,  i have made an bootale usb (Fat32 because uefi doesn;t recognise NTFS) , i have disabled secure boot of UeFI , i have rebooted my laptop following the steps in the link below that i found in this community

http://www.dell.com/support/Article/us/en/19/614293/EN

but after step 2 in the starting the setup category it start windows 8.1 again ....

i have tried everything is there something that i am missing ?

I can boot from my usb at all in other way !!