soundguruman :
No you can't move it. But you can move your personal files off C onto D
Yes, the OS could be moved - and quite easily too, but it's not necessarily a good idea.
eg: image the OS using a freeware or commercial disk imaging tool, restore image to the second hdd, update the boot.ini and then the only thing left to tidy up is the original copy of the operating system. quite a straight forward and simple job and with fast drives, it can be done in a relatively short period of time too.
I am ignoring the question of whether or not it's a good idea or the best solution to the poster's problem, but if moving the OS is useful, then it's quite simple to do.