Step 1. Download windows 7 ( or windows 8 ) iso file.
Step 2. Extract the contents of the iso file using Winrar/7 zip to some folder. ( eg . E:win 7)
Step 3. Now insert USB flash drive.
Step 4. Save the data and format the USB flash drive using NTFS as file system.(We are assuming that your USB flash drive has been labeled as D: by your computer)
Step 5. Open command prompt and type the following commands.
1. E: <enter>
2. cd win 7 <enter>(folder where you extracted your iso file)
3. cd boot <enter>
4. bootsect /nt60 g: <enter>
“Successfully updated NTFS filesystem bootcode” message will be displayed on the command prompt.
updated NTFS filesystem bootcode
Step 6 . Copy the contents of the folder E:win 7 to USB flash drive (D:)
Step 7 . Set your BIOS to boot from USB.