When you delete a file in Windows, the contents of the files are not removed, only its index is removed from file table. The area used by that file is made free for other files to use. There is a high chance of recovering the deleted files using freeware tools like Recuva (http://www.recuva.com/) from Piriform.
You can use many freeware tools available on the internet to securely and permanently delete your files so that they are wiped out from your hard disk and nobody can recover them later. This article shows you how to do so.
SDelete is a command line software tool for Windows, which you can download from https://docs.microsoft.com/en-us/sysinternals/downloads/sdelete. Older version of SDelete (32-bit version 1.51) that works on Windows XP can be downloaded from https://web.archive.org/web/20101207031537/http://download.sysinternals.com/Files/SDelete.zip.
This allows you to permanently delete files or folders, thereby making it impossible to recover any previously deleted files. It also allows you to wipe out free hard disk space.
Download and extract sdelete.exe to Windows folder (in most cases C:\Windows). Then open a command prompt from Start Menu → Run, typing cmd and pressing Enter. When you type sdelete in the command prompt you would see the command line syntax as shown in the picture :
The option -p tells how many times you want to overwrite the file. The option -s tells to delete not only to delete the files in the specified folder but also in its subfolders as well. The options -c and-z are for wiping free space on the hard disk.
To permanently delete a file C:\secret\picture.jpg, give the command : sdelete -p 5 c:\secret\picture.jpg. This would overwrite the file 5 times which is good enough.
To permanently delete a folder C:\secret\, give the command : sdelete -p 2 C:\secret\. This would overwrite the all the files in that folder 2 times and delete them.
To permanently delete a folder and all the sub-folders in C:\secret\, give the command : sdelete -p 3 -s c:\secret. This would overwrite the all the files in that folder and its subfolders 3 times and delete them.
To wipe out free disk space on C:, give the command : sdelete -c -p 1 c:. This would overwrite all the free space on C: one time. This would make it impossible to recover previously deleted files on C: partition.
Eraser is a free tool for Windows which you can use to permanently delete files and folders. It can also wipe free hard disk space just like the SDelete tool.
First of all you have to download Eraser from http://www.heidi.ie/eraser/. Older versions of Eraser that do not require .NET framework can be downloaded from https://sourceforge.net/projects/eraser/files/Eraser%205/5.8.8/.
You can download either the installable version or the portable version. The portable version is such that you will not have to install it but just extract it to a folder and run it from there. You can also copy the portable version to a USB thumb drive, so that you can run it from the USB pen drive.
We will follow the instruction for the portable version. We being by double-clicking on Eraser.exe from the folder in which you extracted the portable version. This would open the main window of Eraser. To add a task, press Ctrl-N or select File • New Task as shown in the picture below.
This would open Task Properties dialog. The first option is to wipe free disk space which would make it impossible to recover any previously deleted file. You can select a partition to wipe the free space of. The second option is to erase a folder and all the files in it. You can optionally choose to delete all the sub-folders as well. The third option is to erase a single file. You can use wildcards to select multiple files with a matching wildcard file name.
You can set the erasing methods from the menu Edit → Preferences → Erasing. You can also you the shortcut key Ctrl-E. It is better to keep the default erasing options as it is. When you are done setting the erasing options, select a task by clicking on it and press Ctrl-R to run it. You can also right-click on the task and select Run.
Conclusion: Before you give your hard drive or computer away or sell it to someone, you should wipe your hard drive and erase all of your personal files. Tools such as SDelete and Eraser can be very useful for these purposes.