Once your pc is booted,which is when you are on your desktop all boot devices and boot priorities are loaded and executed,there is no way in which you can alter the boot device with in an OS,Because the OS is separated from the Bios in some aspects.Now if you are talking about Setting your Boot Device to a USB or a DVD you must use an ISO image to do so,and you cannot boot an 'OS' from a DvD or a USB like you do from the HDD cause you will probably fry its ***.I dont know any software which offers this facility i will let you know more,if i stumble upon something.
