Used disc space is bigger then size of all files. Why?

I recognize the picture on the right. That's from the properties of the c:-drive as told by Windows Explorer.
But what program made the picture on the left, and what does the PHP in the title bar mean?

To see which of the two is right, compare with the output from chkdsk on that drive. That should be equal to what Windows Explorer reports (but I can't check that at the moment, because it reports errors, so I'll have to run at boot to correct those).
That would mean that the program that made the picture on the left didn't include all files. That can happen with programs.

Kees

comparison because of how a file is written to the disk.

During the formatting of a disk the disk is broken up into SECTORS based on the geometry of the drive and the size of each sector by default depends on the physical size of the drive itself. This determines how much drive space is used to store a file. This link breaks it down in detail for you - http://support.microsoft.com/kb/314878

Now you have a drive larger than 2 GB so by default unless YOU or whoever formatted the drive selected a smaller allocation size than the default 4096 bytes (4 KB) that means that each file will use AT LEAST 4 KB even if it is actually only 256 bytes. a file that is 4098 bytes is 2 bytes larger than a file allocation unit so it will require two file allocation units which means that it will take up 8196 Bytes (8 KB) which as you can see is consuming much more disk space than the actual file size accounts for.

Hope this helps you understand that there is really nothing wrong with what you are seeing - one is showing actual file sizes while the other is showing how much disk space is needed for actually storing them.

The best way to minimize slack on your drives it to override the default and specify a 512 byte allocation size when formatting a disk. This assumes a lot of small files. If most of your files are larger a larger allocation size will be faster.