More and more, computers are being sold without CD or DVD drives. This is not a huge deal in most cases, but can cause some concern if for some reason you need to use an external boot device with your computer.
In normal use, the "boot files" (files that allow the system to actually launch your operating system when you press the power button) are located on your main drive. Sometimes though, you may want to start up the computer without launching the operating system, such as if you have a nasty type of virus that is causing problems if you try a normal start up.
Typically this would be accomplished by creating a bootable CD, inserting into your PC, powering up the computer and "telling it" to use that device to start from, instead of your actual installed hard drive. See here for Windows and here for Mac computers.
Usually your CD burning software makes it pretty easy to make a "bootable" CD or DVD (one that can start up a computer, bypassing the main hard drive). But what if you don't have a CD or DVD drive in the first place - as with a newer computer?
In that case, almost all recent computers can also boot (start up) from a USB flash drive. It's not immediately obvious though how you get from downloading a bootable ISO image to creating a bootable USB drive.
An ISO image is just a special kind of single file that can be used to recreate a CD or DVD. A bootable ISO file allows you to recreate a bootable CD or DVD.
One quick solution I recently found - and is very straightforward to use - is a small free program named "Rufus", which will run on any Windows version from XP and later.
All you need is:
a) - the ISO file you want to use
b) - the Rufus utility
c) - a USB flash drive of suitable size
I would recommend downloading the "portable" version of Rufus, no installation is required, just double click the file to run the Rufus program.
Please note that the USB drive will be wiped/erased during the process of putting the ISO contents on it, so don't use one with important stuff on it.
Although Rufus initially looks to have a lot of "stuff" on it's main screen [below], there are typically only a couple of settings you would change. See my comments in red on the screenshot below.
1 - Make sure the USB flash device you want to use is shown under "Device" [top of screen].
2 - Use the drop down to select "ISO" next to "create a bootable disk using", and then click the button to the right to actually select the ISO file you want to use.
3 - Then click "Start", and wait till the program finishes. It's generally quite fast, and of course it depends on how big the ISO file is.
The Rufus FAQ is here.