As said above, re-install the operating system.
Next though, you should download CCleaner and install it. Run the program and at the bottom of the "cleaner" tab you'll find an option that says "wipe clean space." Check that box off, then go down to the options tab. Under "settings" check off the "secure file deletion" option, and set it to at least 7 pass in the drop down box. Now go back to the cleaner tab and click run.
This process ensures none of your data can be recovered. It's always better to be safe than sorry.
Note, it will take a very long time to wipe the empty space. This is normal and to be expected.