HDD used space discrepancy

[Modred189]

So I am planning on upgrading my netbook to an SSD (probably 40gb for budget reasons), so I wanted to try and clean out my existing drive of all non-essential, and largely unused, crap. In doing so, I am having a very hard time getting it down below 40gb, despite getting rid of almost everything.
So I downloaded Windirstat in order to get a better idea of how the HDD was being used. A
And here's where things get odd.
Windirstat shows that I am using 18GB of my C drive, while Windows reports that I am using 42GB.

Why the difference?
Which is correct?
I have a feeling it's obvious but I'm missing it...

 

[Modred189]

No one has any ideas?

 

[InvisiBill]

Hmmm... Not sure, but I get the same thing here on XP x64 (just not as bad). WinDirStat says my C: drive is 7.4GB. Properties on C: says 8.92GB. Properties when selecting all the files and folders in C: says 9.82GB (with a Size on Disk of 8.81GB).

Actually... System Volume Information seems to be compressed (900MB, but 500MB on disk). WinDirStat seems to show the compressed size (the actual disk space used) rather than the uncompressed file size. How much space are you allowing for System Restore?

 

[Modred189]

5.5gb, according to Windows, representing 5% of the HDD space.