Right, first off, I'm from the '00s so although I never got to use {MS,PC}-DOS, I do know what they are, as well as being able to have used Windows 98 SE for a bit when I was younger.
So what I want to know is why does Windows use drive C: for the OS drive, more so, why does DOS do the same? Why not use something like X: (which I can say for a fact that the Windows 7+ install media does for the RAMDisk it creates)?
Way back when, most DOS computers had floppy drives. The first floppy drive was A: and the second floppy drive was B:. Because -- although floppies were absurdly small by today's standards -- some types of floppies were large enough that they couldn't be copied in a single pass through memory (copy original to memory, remove original, insert new, copy memory to new). To make copying a less tedious operation, it was very common to have two floppy drives, so A: and B: were reserved.
When hard drives started to become common, most machines still had at least one floppy, and a great deal of software assumed that both A: and B: were floppies. So, the first hard drive was assigned the name C: to avoid breaking the floppy custom.
Over the years that floppies remained common on machines that also had hard drives, the standard of C: as the first hard drive developed into a widely assumed rule, to the point that even after floppies became uncommon (and now obsolete), C: stuck as the assumed first hard drive.
Just to make things confusing, however, SSDs have started to replace hard drives as the C: drive -- and some software fails to respect that a lot of users prefer that really large installations to go on their hard drive (D:, for example) rather than their SSD.
