To boot a OS using Pendrive format

Using command prompt

  TO Open Command Prompt: Start > All programs >Accessories > right click on Command Prompt or press start+R and type cmd

-First type, DISKPART and hit enter

A new window will appear and execute the following commands in that new window.

-LIST DISK and hit enter
-SELECT DISK 1
-CLEAN
-CREATE PARTITION PRIMARY
-SELECT PARTITION 1
-ACTIVE
-FORMAT FS=NTFS quick
(This process may take few seconds)

-ASSIGN
-EXIT

NOTE:select the disk properly[1 or 2]correctly 

Do not close the command prompt. Minimize it.
Insert window 7 DVD .
Maximize cmd and execute the following commands