Mysterious space filled on hard drive in vista?

i've got a 250gb hard drive with a 10gb recovery partition, so i end up with 220.5gb of space on my C: drive. a program called spacemonger informs me that windows, programs, media and doents (so, actual files) take up 26.8gb (including two files: pagefil.sys @3.6gb and hiberfil.sys@3.3gb) while "unscannable" space uses 30.3gb. what could be using all that space? how can i get it back?