A bootloader would make it easy. I use GAG most often.
But their is no reason why both drives can't be C:. In fact, if it is a clone, then the second drive is already C:. In the NT family, the drive letters are not intrinsic to the partitions, so the second drive will be D: when looked at from the first drive, but C: when you are running from that drive.
Drive letters are kind of an anachronism and really are not too useful. Vista even has the option to dispense with them altogether and just use drive names if you choose. Drive letters may still be useful under some circumstances, but they are ridden with peril. Many people have destroyed their installations by believing that they were partitioning or formatting the correct partition when drive letters are very changeable. Identification is better done by location or size, or both.
Free boot managers:
GAG Graphical Boot Loader XOSL (Extended Operating System Loader) Boot-Us (Graphical or command-line)