Anyone encountered issue with vanishing hdd space on Windows 7 ??

My father has this issue: amount of free and used space doesn't add up to the correct total. The problem slowly but steadily worsens.
I tried to help to the best of my systems knowledge but no cigar.
We checked sizes of everything with windirstat so we are somewhat sure that used space is calculated correctly.
We ran native disk cleaning, the trash is empty and pagefile is set to static size.

Honestly i ran out of ideas, last one is take a peek at the disk in something like gparted but i doubt I'll learn much.

I'm counting someone here will help me...
Google failed me, only devRant can save me now!