How old is this computer? Do you have a lot of files on there, such as video, mp3's, etc? Do you have any file sharing software on there? Did you have a lot of space and then suddenly you lost it all, or was this gradual?
I ask because I am trying to determine if this is possibly due to a virus/spyware or other unwanted program or if this is just because you have a lot of personal files on the computer.