This for me was also a problem when I first tried going about installing Ubuntu 10.10 onto my lap top computer. But as Leeky hinted, I was forgetting about the whole BIOS thing when I made my first failed attempts. Then I was like "Aaron drink a cup of coffee and put a new brain in your head", "duh Aaron, BIOS my man, BIOS"! After I talked trash about myself and came around to it, as Leeky said "F12" went into my boot up area, found that the USB option was now available, due to having the USB flash drive hooked up, BIOS detected it. I then selected the 1st boot device set to USB and whoola.....presto.......things after that fell into place.
After your install is done, simply remove the USB flash drive from the computer. Your BIOS will detect the removal and not give you the option to boot up to a USB device and it will return everything back to normal.