Starting your PC using USB rescue media is not difficult, although it first requires an adjustment in the BIOS (Basic Input Output System). That’s because the BIOS settings include the boot sequence the machine follows when starting up. The boot order tells the machine which devices to search for the software needed to launch the computer and the priority each device in that search.If you’re booting from USB media, you must change the BIOS boot order so the USB device is listed first. Otherwise the computer will load from the hard drive as normal.Start by plugging the thumb drive into a USB port. Then to change the BIOS boot sequence: In the future, your computer will first check the USB port for boot media when starting up. That won’t be a problem, since the BIOS will move to the next device in the boot sequence ... unless you keep the boot media in the USB port. Then the system will launch from that device every time.
For USB images that come from Dell, the usual startup procedure on the Inspiron is Start-F12.  But I sometimes have boot images that come from other sources, and Start-F12 doesn't always work for them.  Is there another way to boot from a USB stick?                                                 start           start-
Booting from a USB stick in general