Most of the time u wud have felt the necessity of installing the windows 7 or vista from a pen drive or a usb drive...
so this is how it is done..:)

ALL THE COMMAND LISTED OUT ARE IN DOUBLE QUOTES. U HAVE TO TYPE THE WITHOUT THE QUOTES
 
 1.) insert the usb drive , open cmd  ,type "DISKPART"
 2.)now type list disk
 3.)see the disk number of ur usb and then type "select disk disk number" 
    for ex:-
if ur disk number is 1 u wud type "select disk 1"
 4.)now type "clean",then type "create partition primary",then type" format fs=ntfs quick"
 5.)after formating completes type "active",then type "assign"
 6.)close the diskpart now copy all the files from windows installation disc/iso file to the root of the usb drive.

now insert the usb drive to pc on which the windows needs to be installed . make sure to select boot device as usb in the bios before booting up the pc. Then carry out the normal installation procedures with faster speed and reliability.
thats all and ur done ..:)