You need to restart the computer and tap Delete or Function 2 or whichever key the system needs to let you into the Setup or BIOS menus. Each BIOS varies but you need to look for Boot Sequence or Boot Order and make sure that USB device is an option. If it is, put it to the top of the list and if it's not there, seek out an entry in BIOS for USB Legacy Support and check it's enabled. Function 10 and Enter key for Yes saves the settings and gets you out.
I have usb external notebook harddisk it has winxp s2 already by ghost program. But I can not boot from it. Can you help me.
If that doesn't do the trick. your BIOS may be of an age where you cannot do it or the ghost utility failed to make your USB stick bootable. Try it in another computer of you can or post back some details of your own system so someone can advise whether that system supprts USB booting.