I've followed the instructions on the Ubuntu website and used the Pendrive software to create a bootable 12.04LTS USB stick but when I try to boot from it my PC says "missing operating system" and continues right into windows. I've tried this on both an ACER netbook and a Dell D630. Both say the say thing. Both appear to recognize the USB drive in the BIOS. I've hit F12 for the one time boot menu and chosen the flash drive but still both PCs say "missing OS". I've also tried the LinuxLive USB creator software and i get the same thing. What am i doing wrong?