Windows 8 Enterprise version allows the creation of bootable Windows on USB device called Windows To Go.
To make a Windows To Go drive, follow the steps below:
Start with a Windows 7 /8 Ultimate/Enterprise or Window Server 2008R2 OS, create a VHD and mount it.
Install EASYBCD so you have a BCD Manager for later use
Format the newly created VHD.
Extract install.wim from the Windows 8 enterprise DVD, note that x86 and x64 versions are different. The file is found in the sources folder on the DVD, should be about 2.7GB in size, this is your installation image.
Download imagex.exe from here. 32bit or 64bit. If you cannot find it here go get it from the Windows 7 Automatic Installation Kit ( big download from Microsoft )
Open up a command prompt , go to the folder containing your imagex.exe and install.wim files and type in "imagex /apply install.wim 1 h:\" - you need to open DOS ad administrator - h:\ is the drive letter of the VHD drive
This should take about 15 minutes, once completed, run EASYBCD and Deploy BCD to drive h:, the VHD drive
Add the OS to the boot menu.