Using existing Windows 7 drive with iMac

I just brought home a new 27" iMac. I have a Mini and a PC that I will be switching over from. I would like to take the PC drive out and put it into a SATA enclosure to use under Boot Camp on the Mac.

Do I need to create a Boot Camp partition on the Mac drive? The only info I have been able to find is on the Winclone site and if I can just run everything off the existing hard drive without installing a fresh OS that would be my preference.