hard drive confusion?

I’m new at all this pc stuff so bear with me here. I had two hard drives hooked up in my pc until I noticed that my computer wouldnt defrag. It was seeing drive c as d and or vice versa. Some files were eventually all over the place and it was quitea mess so I took the smaller of the two out. Now my computer will defrag just fine. Now after looking at my mothers system, she’s having the same trouble with hers and refuses to use her system now out of frustration. Can you tell me the cause of this and or how to resolve this issue?
What are VXD errors? Why do they occur and how to fix them?