Most computer platforms come with a clean up or "de-frag" function that can be accessed by clicking "Start" then "Search", then type in "Disk Cleanup".
You'll get a function to click on that when you do, it will scan your C-drive and tell you how much space it can clear by deleting unnecessary files, temporary files, old stuff, etc. In the pop-up box, will be a list of things you can choose to delete or not, by checking/unchecking, and it will show how many KB's of space they're taking up.
Stuff like "Compress Old files" or "Temporary Internet Files", "Set Up Files" can safely be removed to make your PC run faster.
Also, another good clean up program is this which we use at work:
http://www.stevengould.org/index.php?option=com_content&task=view&id=29&Itemid=70It's free-ware, but you can opt to make a donation to keep it being free when you download it, and it does a little better job than the regular disk clean up that comes on your computer.
Try one of those and you should see faster operation--good luck!