That's one thing I never understood, why my floppy drive was A:\, my hard drive was C:\, my disk drive was D:\, and why of all letters my mapped network drive was Z:\
Please pardon the storytime that follows.
Windows traces much of its filesystem lineage (at least how it's presented to the user) down the MS-DOS line. While the last vestiges of DOS itself had been excised from home computing by Windows XP, Microsoft's first version of Windows NT (based heavily on VMS) for consumers, several legacy issues needed to be maintained because most Windows programs assumed a DOS-like system underneath the Windows interface. Even Windows NT versions before XP needed to maintain this illusion, most famously so that middle managers could use their expensive Windows 3.1/Windows for Workgroups programs in Windows NT 3.5, and Windows 95/98 programs in Windows NT 4.0.
MS-DOS and every DOS-compatible OS, such as DR-DOS and OS/2, had no concept of filesystem mount points or unified filesystems. Every disk drive had to be assigned a drive letter, and Microsoft made the decision that letters A and B would be reserved for the two floppy disk drives. As an aside, there was no established requirement on how networked file shares should be mapped, but Novell Netware admins quickly learned that starting from the end and working backward with drive letter assignments made for the fewest headaches and all but eliminated the chance that a network share would conflict with a local drive.
As another aside, MS-DOS had no concept of nested directories until version 2.0, but convention had already established that programs use the slash ["/"] to indicate switches: "dir /w" to get a wide directory listing instead of the normal one, for example. Using the slash for directory path separators, as Unix did, would lead to confusion, so Microsoft chose the less ambiguous backslash ["\"] instead. That's why Microsoft's operating systems are almost completely alone in using the backslash as the drive/directory/file path separator.
Now, the reason drives A and B were reserved for the floppy drives is because MS-DOS assumed every system had at least two disk drives. If this assumption wasn't made, then it would be impossible for users to perform basic operations like copying disks or copying files from one disk to another. Since most IBM compatibles in the days of DOS were sold with only one floppy drive, not two, this assumption meant that MS-DOS had to emulate two drives: the one physical drive became two virtual drives, with MS-DOS telling users when to switch out the disk for Drive A: with the disk for Drive B: and vice versa. Every version of MS-DOS from 1.0 through 6.22, and every DOS-compatible OS, made and documented this assumption. When hard drives started being sold for PCs, this assumption had to be preserved, so they used the first available letter that wasn't either a real or virtual disk drive: C.
All of this happened because, when Microsoft got the contract to make an operating system for the IBM Personal Computer, they did what has since been recognized as Microsoft tradition: they bought an OS called QDOS from Seattle Computing (QDOS standing for "quick and dirty operating sytem"), which was a clone of CP/M specifically made for the 16-bit Intel 8086/8088 CPU. (The actual CP/M was targeted mainly for the 8-bit Zilog Z80 or Intel 8080 CPU.) Microsoft dressed up and rebranded the CP/M clone and licensed it to IBM for the PC, and the rest is history.
One more aside: One of the most legendary pivotal moments in computing history was IBM's decision to commission Microsoft instead of Digital Research for the PC's OS. Had IBM gone with Digital Research instead, few things would be different today. IBM would've shipped the actual CP/M itself instead of a clone trying to pass itself as not a clone. Today's PCs would be using a GUI called GEM instead of Windows, with the same CP/M legacy issues.
Linux, Mac OS, and Unix also have some rather ancient legacies, some far older than CP/M, but of the three only Mac OS's is traced to an origin in low-power home/office computing.