Although the letters Windows assigns to your disks are somewhat arbitrary, the operating system will make sure they never conflict. However, while I haven't dual-booted two versions of Windows since mid last decade, I distinctly remember Windows XP assigning the boot partition the letter "D" on one installation (dual-booting Windows XP SP1 with the pre-existing Windows 98 SE installation). Up until that point I thought that Windows always automatically assigned "C" to the boot disk regardless. Since I haven't tested this in a very long time, SharpGreen might be correct that modern versions of Windows will always assign the partition they are running from the letter "C".
If you want to assign a different letter to any of your partitions, it is fairly easy to do using the Windows Disk Management utility. Just right-click on the partition you wish to reassign, click "Change drive letter and paths...", and use the resulting dialog to "Add...", "Change...", or "Remove" drive letters from the selected partition. Since drive letters cannot conflict, you will need to remove a drive letter that is in use before assigning it to another partition. Older versions of Windows did not allow you to reassign "C", although all other partitions could be reassigned. It looks like that might have changed sometime between Windows 2000 and Windows 8.1 based on the "Change Drive Letters and Paths" dialog I based this reply on, but I have no intention of potentially screwing up my only Windows VM to find out.