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.
My mom's computer is running very slowly. I looked at her C and D drive and evidently everything has been stored in D over time so it is now at 94% full and the C drive is virtually empty. So I have two issues. I cannot open the D drive to see what is there and I do not know how to move everything to the C drive so she uses it now?
This is an HP Pavillion with XP and 8 gb on the recovery and 188 gb on the C.