1) Go to command prompt

2) Type diskpart

3) Put your pen drive on the PC with at least 4 GB capacity 8 GB will be better. Then type list disk

4) Select the disk which is your pendrive.

5) Now you have to run different commands
> clean
> create partition primary
> select partition 1
> active
> formatfs=fat32
These steps are shown in the below image

6) After format is complete (Format may take long time):
> assign
> exit

” The image below shows the detailed steps:

7) After the format is complete copy all the contents of windows 7 DVD and paste it in USB drive. Now your USB is device is ready to boot windows 7.