You probably know that when you delete a file, you're not really removing the information from your storage device. Instead, you're designating the space taken up by the file as available for storing new data, should it be needed.
A properly motivated person can recover the deleted information. In fact, data recovery tools such as the free Recuva exist for this very purpose. To keep prying eyes from perusing your deleted data, you need to write over the digital bits that comprise the file. That's where secure-erase utilities come in.
(By the way, the process of recovering partially or superficially deleted information is known as data remanence.)
There are plenty of government standards for ensuring that deleted files are truly erased by writing over the data several times (scroll down this Wikipedia page to "Standards" for more info). But for most people, a single pass of random zeros and ones will suffice.
Among the many programs that offer to securely delete the files on your hard drive and other storage devices, two freebies stand out, though in terms of approach, they're polar opposites: Heidi Computers' Eraser and SDelete from Sysinternals.
Drag and drop to wipe out files
Few file-wiping utilities are easier to use than Heidi Computers' Eraser, which has Explorer-like windows into which you can drag and drop the files you want to securely delete. You can also schedule an overwrite of all the available space on a disk or erase individual files in Windows Explorer by right-clicking them and choosing Erase.
Another right-click option added by the utility is Erase Secure Move, which wipes the selected file from its current location and lets you place it onto another storage device. You can perform the same secure file move by right-clicking as you drag the file to its new location and choosing Secure Move with Eraser when you release the button.
Take the command-line approach
When it comes to free Windows utilities, they don't get much better than the great tools from Mark Russinovich's Sysinternals, which are now the property of Microsoft. Among the programs is SDelete, a command-line utility that securely wipes files, directories, or the unused space on a disk.
First, open a command prompt: in Vista, click Start, type cmd, and press Enter; in XP, click Start > Run, type cmd, and press Enter. At the command prompt, type sdelete, enter the operator for the desired action, and specify the file, directory, or drive to be acted upon. How straightforward can you get?
In addition to wiping files and directories, you can use the sdelete command to overwrite the free space on a disk and specify the number of passes used to obliterate the data. You'll find more information about the program on Microsoft's TechNet site.