WinToUSB is a free software allows you to install and run Windows operating system on a USB hard drive or USB flash drive, using an ISO image or CD/DVD drive as the source of installation. WinToUSB also support creating bootable WinPE USB drive, it can help you to transfer the contents of WinPE to the USB drive and make the drive bootable.
WinToUSB's key features include: Easy-to-use wizard interface that provides step-by-step instructions for installing Windows/WinPE on a USB hard drive or USB flash drive. Install Windows/WinPE from an ISO image or CD/DVD drive. Support for Windows NT 6.x OS family (Vista/7/8/8.1/2008/2008 R2/2012/2012 R2) and WinPE 2/3/4/5. Use any edition of Windows 8/8.1 to create Windows To Go USB drive. Support for MBR and GPT disk layouts. Don't need install WAIK (Windows Automated Installation Kit) or WADK (Windows Assessment and Deployment Kit).
Important notes: Windows Vista/7/2008/2008 R2 does not have built-in USB 3.0 support, so Windows Vista/7/2008 (R2) will have to be booted from a USB 2.0 port. USB flash drives are very slow. It takes a long time to install and run Windows from a USB flash drive, highly recommend using a USB hard disk. Windows To Go drives can be booted on different computers, so you can carry it to anywhere and use it on any computer.Windows 7 is not completely portable. You may have activation and driver problems when booting on different computers.