If you want a flash drive to be bootable, you have to use a utility like diskpart to do that. Then copying an install DVD to it will make it act as bootable install media for Windows.

A. Make a bootable flash drive by opening a Command Prompt, 'Run as Administrator', to run these commands:
diskpart
list disk  ('list disk' to determine number)

select disk 2
clean
create partition primary
select partition 1
active
format fs=fat32
assign
exit

B. Copy DVD to the Flash Drive
xcopy d:\*.* /s/e/f e:\

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Mark L. Ferguson MS-MVP


http://blog.powerbiz.net.au/fixes/using-diskpart-to-create-a-bootable-usb-of-windows-8/