WinToUSB is free software that allows you to install Windows/WinPE on a USB hard drive or flash drive and boot from that drive. The program has an easy-to-use wizard interface that provides step-by-step instructions for installing Windows on a USB drive. The program is compatible with Windows 7, Windows Server 2012 and Windows 8. In particular, in the latest version 8.1, it is possible to create a live boot system like those used by LiveCDs for some Linux distributions.
The software is fully secure so that we have a personal bootable USB version with us on the go when we need to use public machines. With this program, any user can copy the ISO image of the original CD to an external drive to make it an installation source instead of using a physical drive. This program is a great way to store the Windows installer on USB, so you can always have it with you or run it from computers that no longer have a CD reader. Simply copy it to a removable drive and insert it into any computer. WinToUSB also allows you to store data and save preferences for reuse. Moreover, if what you are looking for is to create a WinPE bootable unit, you can also transfer it to USB with this application.