Quote Originally Posted by Oli
Make sure you're folders are set to show hidden files (Tools>Folder Options>View>Show hidden files and folders)
That's what I was going to suggest. Maybe somebody has downloaded a crap load of porn on your pc (a brother maybe?) and hidden it.

Try going through every folder in My Documents and seeing what you don't need. Delete it. Do the same for the Program Files folder, but make sure to leave it alone if you don't know what it is... or search on Google to find out. Run AdAware and Spybot Search & Destroy, and trying searching for and downloading some reg tools.