You can only do it by the C, there's no other way.  I know with my toshiba laptop I have a recovery disc that I have to use if I want to format my computer (I'm quite sure there is a partition on my laptop where it installs it from).  You might have a simular thing, but you will need to use the CD, making sure the boot order is for the C drive before the HDD and that the CD is in the drive before restarting the computer.  Good luck and have fun with th hours of re-installing ahead of you 
