Insert a windows 7/vista/xp disc into your diskdrive
Now click on START > RUN or press [windows] + R to launch run dialog box
Type cmd and press [enter]
Connect the pen drive and format it in NTFS format
In command prompt window type diskpart press [enter].the command utility "diskpart" is powerful disk partitioning utility which is included in windows.it can be used for creating,removing and shrinking partition.for details type /? in diskpart prompt will bring you list of its commands
Type "list volume" and press [enter]. This will list all the volumes with their volume no. connected to your computer,for example my pendrive has assigned drive letter (I:) and its volume no is "6".
Type "select volume <pendrive volume number>" and press [enter] (here it is 6)
Type "active" and press [enter]. This will set an active flag on the pendrive volume.
Exit from diskpart using the "exit" command.
Now in command prompt navigate to your cd/dvd drive by typing <disk drive letter>: and press [enter].for eg:type d: and press [enter].
Type cd boot
Type "bootsect.exe /nt60 <pendrive letter>: ".The bootsect utility will transfer the boot sector to the pendrive so that it becomes bootable.The "/nt60" parameter is to specify that the windows vista-compatible boot-code needs to be applied for windows vista or above as win 7 while for windows xp or below replace it by "/nt52".
If you get an error message saying "access is denied",then run cmd as administrator by right-clicking on cmd icon and select "Run as administrator" for windows 7 and log in as administrator in windows XP.
Now copy all the files from the windows vista/7 dvd to he the root of pendrive.
Now that's all ,you have a bootable pendrive with the windows vista /7 setup to boot a computer that doesnot have any disk drive.
Connect the pen drive to the usb port of computer to be booted and press [esc] or [F12] or [F8], when BIOS of system comes up.
Here select from boot options "boot from usb storage device" and the booting will start....