Hello !The problem is simply that your have to plug your USB key or your USB CDDrive on a standard USB 2.0 port (black port on the left of the machine), and not in a new-driver-needy- USB 3.0 port (blue port). Try to change the USB port for your USB Key and it will work ! V.
I am trying to install Windows 7 32 bit on my laptop but i am facing a problem while installing. Once the installer boots up and i click on "Install Now" button, an error pops up saying that:  A required CD/DVD drive device driver is missing. If you have a driver ...,please insert it now. This is despite the fact that i have redownloaded the image several times and also tried to install windows from USB, but still facing the same problem. I have googled about the problem and have came to know that it might be resolved by upgrading firmware of your DVD writer. 
Problem in Installing windows 7 on Dell Inspiron n5110