If you suffer a bit of OCD you'd probably have been frustrated as to why Windows title's your main hard drive 'C' and not 'A' by default, but there is actually a very good reason for this.
Way back in the day, prior to Windows XP when DOS was still commonly used, computers didn't even have hard drives, the entire computer's operating system was run off of a floppy disk. Therefore the main floppy disk drive was titled the 'A' drive, followed by another floppy disk drive to run other programs which were titled the 'B' drive.
When dedicated internal hard drives became more popular in computers, these were considered secondary to the floppy drive slots and was therefore titled the 'C' drive.
When Microsoft released Windows XP, floppy drives were still used by many people at the time and was still categorised as the 'A' drive. But as time went on, floppy drives started phasing out, but many programs still used the 'C' drive as its main file directory which hasn't changed since hence why the 'C' drive is still the default directory for your PC's main hard drive.
Take a look at the video below by the YouTube channel: Tech Quickie on Why Your Hard Drive Uses The Letter C...