Download windows 7 usb dvd tool from link given below

http://images2.store.microsoft.com/prod/clustera/framework/w7udt/1.0/en-us/Windows7-USB-DVD-tool.exe

Run it.

browse to the source of windows 7 iso image
click next and then specify the destination drive.
it’ll ask to format, just accept everything and then proceed.
Finally your windows 7 bootable usb pendrive will be ready.