Can a USB HD be boot like Window CD?
Do you mean boot from an image on a USB drive to the PC? Yes you can.
Change your settings in BIOS to make the USB drive the first item to be searched when booting.
The reason I'm asking is because I wanted to format my laptop but the CD ROM drive is broken and I'm wondering if I can make my USB External HD as a window cd to boot, format and install new window...
No. The boot record for a CD is different from a hard drive. You will not be able to make a bootable external drive that acts like a Windows CD without some real down and dirty modifications.
The best you'll be able to do is take the drive out of your laptop and to another system. Use a USB or 2.5" to 3.5" IDE adapter and format the drive using FAT32. Copy the contents of the Windows XP CD to the hard drive in a folder called 'INSTALL' (for instructional purposes). Reinstall the drive in the laptop and boot from a floppy diskette (www.bootdisk.com for a Win 98 diskette image). Run the XP setup program. You can do this by typing C:\Install\i386\winnt.exe
The setup will be painfully slow, but it will begin. You'll also have a FAT32 drive, which is a nasty thing these days. You can convert it later to NTFS, although that has its short comings as well. :\
Thank Rick I'll follow your instruction and hopefully it will work...
Laptop doesn't detect USB floppy, I think it because I format the drive and there is no driver for usb? so what can I do?
No, it has nothing to do with drivers. Your BIOS (and most recent ones do) supports booting from USB floppy regardless of what's installed in the system and it does it on the lowest of levels.
You may need to change your boot order in the BIOS setup. Perhaps there is a boot menu (usually press F8 or F10 during the BIOS screen) and you can select it manually. And its possible your boot diskette is messed up.