V prvom rade si skontroluj obsah toho usb podla odkazu,potom sformatuj cely disk tak ako treba
http://serzovo.wordpress.com/2013/01/29/windows-8- downgrade-na-windows-7-a-secure-boot-problm/
http://www.chip.cz/trendy/instalace-windows-7-uefi /
http://www.sevenforums.com/tutorials/26203-convert -gpt-disk-mbr-disk.html