So what's that d:-drive? All you tell is about a (usually hidden) recovery partition and a c:-partition. If you can't open that d-drive, why do you think your mom can? If she couldn't open it, she couldn't store things on it, could she? And, by the way, why can't you open it?

I think there's some other reason for the PC being slow.

Moreover, I wouldn't call 25% of 188 GB "virtually empty". It's not full, but certainly it's not unusual for a mom's computer.