also, there are things known as hidden files
.. most of the time thats where everything hides.
1gig of space on the windows partition slows down computers nowdays.. I'd say check these folders for stuff to delece (under xp)
c:/documents and settings/your username(or other peoples)/local settings.. Look around in these directory's for folders like "temp" and "temporary internet files".. they could be taking up a decent amount.
You could always go out and buy programs that do this for you.. like The Ultimate troubleshooter v3