80 GB of HDD space gone

@Gaz: No, i don't have a .old file
@Seekermeister: I've not yet downloaded and installed the O&O. I'll try it tomorrow
@omegatron: I was already using CCleaner
@alphanumeric: unfortunately the disk management didn't show me a hidden partition.
@Jonathan_King: I tried WinDirStat this is what i ended up with; it still says I'm only using 81.1 GB (the language is dutch: "<bestanden>" means "files") Edit: I also see now that the WinDirStat only reads 91GB

Check the system restore area.

Right click on Computer, Properties, System Protection, Highlight C and click on Configure. You will see how much space is being used.

Issith said:

oh God i feel so stupid.

Nah...they don't exactly put it right there in front of you. At least in Windows 7, there is a GUI to figure this out. In Vista, you had to run a command-line tool...which tons of people didn't know about.

Issith said:

I guess if i delete all the previous files and set the usage to about 20 GB instead of 68GB I will probably regain 48GB

Yes, that is correct.