hey benh
what i would do if i were you would be look in your folders for any downloads that you clicked save for instead of open because they are no longer needed after you install them and take up a lot of space.
run a system clean up/defrag
check your "add/remove programs" and sort them by date last used and frequency used and delete/uninstall any of them that you KNOW you dont need. like if you have and old game or word processor that you can toss. a good rule to stick to is if you dont know what it is dont delete it.
hope this helps