First we will need to mount the content of the Windows 7 DVD ( or the ISO file ) to the external Flash Memory. Open Command Prompt ( cmd ) and write the following commands:

1. To open Command Prompt , click on Start > All Programs  > Accessories > Command Prompt
   or press windows key + r. 

2.Type diskpart

3.Type list disk , with this command , all hard disk attached to your machine will be listed. My machine internal Hard disk is 75 GB and my flash memory is 8 GB, as shown below :

4. My USB Stick sown above with a Disk ### as Disk 1, so type select Disk 1

5.Type clean, this will clear the configuration information, or all information, off the disk.

6.Then type create partition primary the partition by typing : select partition 1

8. Set it as active by typing : active, this will mark the current basic partition as an active boot partition.

9.Format the partition with a Fat32 File System by typing : format fs=fat32

10.Assign a letter to the flash disk, type assign letter=v , where v is a free drive letter

11.Exit Diskpart by typing : exit

12. Now we need to copy the Windows 7 Installation files to the USB Stick.
Type xcopy d:*.* /s/e/f v:
where d is the drive letter of the DVD drive that holds the Windows 7 DVD, or the Drive letter of the path that you are saving in it the ISO file of Windows 7, and v is the drive letter that you assigned to the external USB Stick in the previous step.

13. Plug the USB stick to the machine you want to install Windows 7 to, and reboot/or start it up. Make sure that USB booting is set in the BIOS prior to your internal hard disk.

Once booted, an animated logo for Windows 7 will be displayed and below it will show you that Windows is starting