Boot from external USB2-HD

Hi! I was just wondering if I can use my external USB2 80 GB Harddrive to be the boot-drive... I installed XP and changed the boot drive to that USB-HD. Installation went well but now that the system wants to boot a fatal error & bluescreen appears saying that windows stopped to boot to avoid harm to hardware...

Is there another possiblility to boot from external drive (I don't want to install large games on the internal drive, therefore I prefer to have some programms/games installed on the external drive. Thanks in advance!