Hi,
your space is probably taken up by system restore files and/or temporary files created by your browser or pc as it's used on a day to day basis. Open the 'Run' app and type in sysdm.cpl and press Enter. Look across the top for 'System Protection' and click that. If you click on the 'Configure' button you can see how much space has been given over to the system restore set up. If need be reduce it although be aware you'll probably lose some restore points.
To be honest your HDD is too full. Ideally you want to leave around 25% of the drive free for it to work effectively. You've probably noticed your machine isn't running as quick as it was? That will be down to the HDD and lack of space. Try this app, it's free and will remove your temporary files as well as empty the recycle bin:
http://www.piriform.com/ccleaner/download