Notebook Operating System and Recovery

So my grandma bought this notebook a while back and never used it so she decided to give it to me. The first issue is that she doesn't remember her account password only her PIN, so I can sign in but can't change much. We've done the password recovery but they wouldn't reset her account.

Anyways, the real issue I'm posting about is that my c: drive constantly fills up almost to max even though I have no personal files on there. I've gone through every section of file explorer and deleted, moved, and compressed everything I could. I have run multiple disk checks that have said nothing is wrong. I've tried a few things I found on Google to type into the command prompt with no luck, I've tried shrinking other partitions to add size to the c: drive but couldn't shrink anything else, I've gone through the defragment and optimize app with no changes. I've tried pushing a few different buttons right when you turn the computer on ( I apologize for my uneducated computer terms) with no changes. I can't do a system recovery because there is not enough room on the drive. I can't do a factory reset because my grandma can't figure out her account password and for that same reason I can't make another admin account to bypass it.

Are there any other options for this computer? Any suggestions would be greatly appreciated! I would add screenshots but have no memory to store them.