I think there's a couple of things you can do.
Go to Start - My Computer - then right hand click the C drive and from the popup menu choose Properties. Firstly click Disk Cleanup which should get rid of any unneccessary rubbish on your drive.
Having done that click the Tools tab and click 'Check Now' and click the two boxes. As you retreat click 'yes' in the box.
Do this for each of your hard drives then when finished, reboot. If you haven't used this option before scanning the drives may well take a few hours. I do mine overnight.
Hope this helps.