Files that are deleted in Windows are not really deleted--just the links to where the files are located on your hard drive are removed. Despite allowing you to use the space for other files again, the actual information is still lurking on your disk space. In other words, a knowledgeable computer user would be able to locate enough information to restore a file you wanted to keep private. Luckily, the ever-useful program CCleaner provides a method to permanently remove any deleted data from your hard drive.
Step 1: Download and install the latest version of CCleaner.
Step 2: From the main CCleaner window, click on the "Tools" icon to the left
Step 3: Select "Drive Wiper" from the list that appears in the new pane.
Step 4: In this pane, select "Free Space Only" from the first drop-down box, then select the number of overwrite passes on the free space.
Step 5: Select the drive with free space you want to clean and click Wipe.
The number of passes you have CCleaner make is up to you. One pass will likely be sufficient, but if you're really serious about security and privacy then you'll probably like to do more passes. The time this process will take to complete is based on the number of passes you choose to make as well as the size of your hard drive.