Can I use BootCamp on an existing Windows partition?
This is a whole bunch of links so I'm not really sure it qualifies as an answer...

Windows -> Parallels - possible with the Parallels Transporter Agent software - http://kb.parallels.com/uk/115007

Parallels -> Bootcamp - http://www.tylercrumpton.com/?p=90

Windows straight to Bootcamp [needs WinClone, not free] - http://twocanoes.com/winclone/support/migrating-a-real-pc-to-boot-camp-with-winclone-4