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I believe rEFIt lets you boot non-OS X partitions from USB. After that you can probably Google the rest - http://google.com/search?q=boot+windows+from+usb

As for Parallels, sure you can save the Virtual Hard Disk file on a USB drive instead of your Main HD. (Note: This is not Booting off the VHD, you still have to use Parallels inside of OS X)
 
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