I bought and built my computer and installed Windows 8 on it. Everything was fine (never really checked my system's hard drive though).
Whenever I install huge software (i.e games, etc.) I do that on another partition (same drive). I only install tools (compilers, video converters, media player and other file format reader, etc.) on the system partition.
Some info about my directories (size - on disk size)
C:\Windows 13.9 - 14.1 GB both Program Files (together) 7.78 - 7.98 GB C:\Programs 1.09 - 1.12 GB C:\Users\Default\AppData 87.6 - 236 KB
My whole C: partition is about 55.5 GB and as you can see in the picture all files/directories use together about 28.1 - 28 GB of space (hidden folders such as ProgramData are visible and selected as you can see). However I only have about 7.43 GB of free space.
Is there a particular reason for that?