Ubuntu install went bad. Is there any way to wipe partitions?

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http://askubuntu.com – I recently installed 12.10 but found out that I had stupidly downloaded 32-bit by accident so I tried to downgrade to 12.04 but didn't succeed. I didn't use wubi so I can't boot into windows to kill the partitions and perform another install. Apparently, I can't just go ahead and delete the root partitions either. I've heard you could do it from live usb but I haven't had the time to try (H/W and shenanigans). Could somebody proficient in the ways of linux help me out with steps? Thanks so much. P.S. I don't want suggestions like 'why don't you...' because I'd MUCH prefer to wipe and re-instal (HowTos)