Here is the explained tutorial for you,how to make USB bootable in windows 7.
Its very very useful to make bootable USB and Install Windows from it.It reduces the time of installation (In my case it was 40% faster than CD ROM).

1- Click on Start and Search CMD

2- Right click on CMD and choose Run as Administrator.

3- After that type “diskpart” (without the quotes)

4- DISKPART will be opened,Now type “list disk” it will display all drives attached to the PC.

5- I used 4 GB Flash Voyager,and Select disk 1 (its in my case,you should check before selecting your drive)

6- Now type “Clean” and it will display the success message.

7- After cleaning now type “create partition primary”.

8- After that type “active”.

9- In this part we will format our drive to FAT32, just type format fs=fat32 quick (quick to format the drive quickly)

10- After successful formatting now type “assign” to assign a letter to that drive.

11- You will get a pop-up box showing the letter of this drive to open.

Discuss here:

http://forum.paksharez.com/thread-how-to-make-usb-bootable-in-windows-7