If you know enough about computers to use a web browser (presumably since you've posted this question), then you will be able to install this free program:
http://windirstat.info/
See the download section, get the setup file for all Windows versions. Once downloaded, double-click to install.
You can then run it. After a minute or two, it'll present you with a brightly coloured picture, showing the contents of your hard drive. The big squares are big chunks of your space being used. You can then decide if you want to remove that stuff or not.
(I'd try this before letting anyone else play with your computer unless you trust them implicitly.)