I have a Dimension 4600, and it has the latest available BIOS (A12), but so far I have not been able to boot from my Flash drive.
This site gives a lot of information on doing this:  http://www.pendrivelinux.com/
I suppose another question would be: Does my computer support booting from a USB flash drive?  It would seem so being that it is on the boot menu when I use F12 during booting.
Can anyone shed any light on this?