Dear FriendZ....
Plz take care of ur bootable usb when u plug it into the usb port, if u r using "usb disk security" bcoz it automatically removes the boot file.....
So 1st open "usb disK security"
open "usb shield" from left menu
and uncheCk "delete threads automatically"
before plugging ur any bootable usb to computer....bcoz "usb disc security" software will delete every boot file, considering it as a thread, and ur usb will not remain bootable....

Thanxxx