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If we don't have or don't want to install a specialist boot manager, we can get a computer to boot from a removable device by directing the bios of the machine to select it as the device to be booted. We can do this either by entering the bios setup of a machine and resetting the boot order so that our device is first on the boot list, or we may be able to use a built-in bios boot menu that we can call up just as the computer is coming to life. These boot menus have been a common option on most PCs for several years now and are accessed by pressing a particular key during the early stages of bootup. By far the most likely keys are F12 or F11, but it could be another F key or the Del or Esc key, or even a combination of keys. Look for some brief instructions on screen at computer start-up telling of the keys to press to display the boot menu, or to enter the bios setup. Sometimes there are no on screen instruction, or they are too fast to even appear. Sometimes the bios boot menu won't be available until an option for it has been enabled in the bios setup.

There is huge variation across different makes and models of PCs so we can only suggest that you check any manual you have or you search the net for information on the 'bios boot menu' for your particular machine. Using a bios boot menu is a one time only event and is preferable and safer than regularly messing around in the bios to reset the boot order. If you do an on-line image search for Bios Boot Menu you should see plenty of examples showing the huge range of different boot menus and bios screens there are. Here is just one example - Phoenix Award BIOS

Note that on store bought Windows-8 machines in full EFI and fast-boot mode the keyboard may not even be initialized until Windows loads, so we will need to go through the new Boot Options Menu.
www.youtube.com/watch
MSDN - Designing for PCs that boot faster than ever before.

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SD, SDHC and SDXC Flash Memory Cards.
Most PCs now have memory card slots, but its only recently that bios support for booting from them has been coming through as standard, so you need to check that your bios boot menu will see them and offer them as a boot option.