The Cipher program can be used to remove any erased data from unused disk space. The command, Ciper /w:C: is used to perform this action and can securely erase data that you once had on your computer. While this command is relatively safe, you should always have a backup of your critical data.
If you want to do this to make sure files are deleted properly you have to remember a couple of things. Only do this when the computer can set idle for several hours. Another thing you have to remember is that if you interrupt this process, the Ciper command can fill up your hard drive with temporary files. If you have to interrupt this process, you may see you have a lot of space that is suddenly missing. Use Space Sniffer to find where the temp files were written so that you can erase them.
You can securely erase other drives by replacing the C in the command above with the drive letter of your choice.