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:
select disk 2 ('list disk' to determine number)
create partition primary
select partition 1
B. Copy DVD to the Flash Drive:
xcopy d:\. /s/e/f e:\