Another easy way ..
Download Rufus v1.1.7 without FreeDos Support (154KB) from Rufus - Create bootable USB drives
Select your pendrive.
Select File System FAT32 or NTFS (I selected NTFS since my Win 7 ISO was 5GB+. If your Win 7 ISO is less than 4 GB you can select FAT32)
Give a New Volume Label.
Check Quick Format.
( If you want to check the pendrive for bad sectors, check Check for bad sectors and select the number of passes.)
Check Create a bootable disk using ISO Image. (select ISO Image from the dropdown box.)
Show the location of your Windows 7 ISO.
Wait for the writing to complete.
Your bootable Win 7 install USB pendrive is ready.