well first of all you need and Iso image of the windows 7 installation disk.
You then will have to go to the Microsoft website where there is a tool to copy the image to a usb flash drive and make it a bootable usb device image.
Since the Microsoft software never worked for me I use this method.
Note: not saying that software doesnot work, I just wasnt able to make the drive bootable. A lot of people that I know have tried it. A lot succeeded some failed like me.
So here it is
1. Plug-in your USB flash drive to USB port and move all the contents from USB drive to a safe location on your system.
2. Open Command Prompt with admin rights. Use any of the below methods to open Command Prompt with admin rights.
*Type cmd in Start menu search box and hit Ctrl+ Shift+ Enter.
*Go to Start menu > All programs > Accessories, right click on Command Prompt and select Run as administrator.
3. First type DISKPART and hit enter to see the below message.
Next type LIST DISK command and note down the Disk number (ex: Disk 1) of your USB flash drive.
4. Next type all the below commands one by one. Here I assume that your disk drive no is “Disk 1”.If you have Disk 2 as your USB flash drive then use Disk 2.Refer the above step to confirm it.
So below are the commands you need to type and execute one by one:
SELECT DISK 1
CREATE PARTITION PRIMARY
SELECT PARTITION 1
(Format process may take few seconds)
Don’t close the command prompt as we need to execute one more command at the next step. Just minimize it.
5. Next insert your Windows 8/7/Vista DVD into the optical drive and check the drive letter of the DVD drive. I assume that your DVD drive letter is “D” and USB drive letter is “H” (open my computer to know about it).
If you have an iso image of windows 7, mount the image with a software like daemon tools, alchohol 120, power ISP, etc.
6. Maximize the minimized Command Prompt in the 4th step.Type the following command now:
D: CD BOOT and hit enter.Where “D” is your DVD drive letter.
CD BOOT and hit enter to see the below message.
7. Type another command given below to update the USB drive with BOOTMGR compatible code.
BOOTSECT.EXE /NT60 H:
Where “H” is your USB drive letter. Once you enter the above command you will see the below message.
8. Copy your Windows 8/7/Vista DVD contents to the USB flash drive.
9. Your USB drive is ready to boot and install Windows 7/Vista. Only thing you need to change the boot priority at the BIOS to USB from the HDD or CD ROM drive. I won’t explain it as it’s just the matter the changing the boot priority or enabling the USB boot option in the BIOS.