Difficulty installing Windows 7 Ultimate

I have just bought a dell inspiron 5379-17 which is Ubuntu preinstalled. I want to install Windows 7 ultimate.I have got the DVD. but the PC has no Optical drive. So I have to boot from a USB flash drive. I have created a bootable USB drive carrying the ISO file. Many attempts at installation have failed. The installer asks for a drive which it says is missing.Please guide me