[ubuntu] Ubuntu installation from USB doesn't work.

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http://ubuntuforums.org – Yesterday I installed Ubunutu from a USB, but not the latest version and I accidentally downloaded 32bit instead on 64bit. Today I downloaded the latest build of Ubuntu in 64bit, created a bootable USB, changed my BIOS to boot from USB, but whenever I start the computer up it just loads Ubuntu straight from the HDD. Why is this happening? Thanks. (Hardware)