I can't tell if you did that. It's a common issue that we all wish Microsoft would nail down.
As to booting from said drive, unless someone tweaked the BIOS or the drive has failed horribly it should work fine.
I have a Toshiba U305-7467 running Vista Premium Home Edition SP1 and yesterday I turn it on to see that I have no optical drive listed in my computer. I go to device manager, and there's no optical drive their either. This is not the first time this has happened, the first time I had to delete my upper and lower filters in the dvd drive registry. It seems that alot of these issues started after updating to SP1 version of Vista. Im at a loss, i was going to just go back to factory, but i would need the drive for that, and i read rebooting from a usb optical drive is a nightmare...I need help! any ideas??