1.Connect your thumb drive to your PC 
2.Right-click “Command Prompt” under Programs > Accessories and select “Run as administrator.” 
3.Find the drive number for your flash drive using these commands 
diskpart 
list disk 
The number of your thumb drive will be in the list. 
4.Format the drive using the commands listed below (we’ll assume your drive number was 1 in this example) 
select disk 1 
clean 
create partition primary 
select partition 1 
active 
format fs=NTFS 
assign 
exit 
5.From the command prompt, browse to the folder you copied the contents of the Windows 7 ISO to and then to the boot folder. If you are using Vista it should look something like 
cd c:\Users\username\Desktop\Windows7\boot\ 
6.Now we will use bootsect to make the USB a bootable NTFS drive that is ready for the Windows 7 Install Files. 
bootsect /nt60 x: (assuming X is the drive letter of your thumb drive.) 
That’s it, you can close the command prompt, the USB drive should now be bootable and ready to go.