You'll have to install the OS on the new drive (to get it to be c:) and then you can simply change the letter of the secondary drive to whatever isn't already assigned (after you boot up). You shouldn't have to reformat the second drive or anything, windows will let you access everything. But you'll have to reinstall all your programs and stuff.
I think there are ways to throw the new drive in and make it take over without installing the OS and everything but you'd be asking for problems doing it that way.