Here's what you should do:
Go to Start
then Programs
then Accessories
then System Tools
Then, click on Disk Cleanup. Let the computer do it's thing.
After that, go back to System tools and click Disk Defragmentation. Again, let the computer do it's thing. It may take a while if you haven't done it before. This will compartmentalize all your miscellaneous disk fragments and free up more space.
Also, regularly delete cookies and temporary internet files from Internet Options in the Tools menu in the toolbar. Another way to keep junk from piling up on your hard drive.
Doing all this can also help your computer to move faster.