As Witness My Fury suggested, right click on your 'Recycle Bin' icon on your desktop and then choose 'Empty Recycle Bin'.
If you don't do that, files that you think you have deleted, have actually been moved to the Recycle Bin and continue to consume space on your hard drive.
One piece of software that I find effective is Tony Arts - EASY CLEANER. In addition to identifying unnecessary files on your drive, it also makes it easy to clear the browser cache and also identify programs that load on startup which usually slow the operation of the computer. Often, there is little benefit for many of these programs to run in the background so you may easily delete their entry in the startup file.
Easy Cleaner needs to be run by the computer's Administrator. After installation, right click the Easy Cleaner icon and choose 'Run As Administrator'.