Here are steps to create bootable USB Flash disk with Windows7.
1. 4GB or bigger USB Flash Disk needed
2. Put your Window 7 Instalation DVD into your computer DVD mechanics. If DVD is not readily available, you may download ISO image, for example, Windows 7 Home Premium x64 with SP1 (bootable) from http://msft.digitalrivercontent.net/win/X17-24209.iso and burn new DVD.
3. On your Windows 7 computer (with DVD in D: and USB Flash in E:) do:
DISKPART>select disk N
DISKPART>create partition primary
DISKPART>assign //letter drive e.g. E:
D:>bootsect /nt60 E:
D:>xcopy D:\* E: /E
4. May sure in BIOS, that computer is able to boot from USB Flash drive. It’s also handy to have picture of computer MS Windows certificate.