Where does all that hard drive space disappear? Windirstat will draw you a picture (http://windirstat.info/ or on Linux http://kdirstat.sourceforge.net/)
This is the 78G windows-partition on my laptop. The size of pagefile.sys and hiberfil.sys probably scale with the amount of RAM (I have 4G). Matlab is just huge (and bloated?). I wonder if an 18G windows-folder is about normal for win-7 users, or if I have a lot of temporary stuff and installer-files etc. hiding in there?