When you a delete a file, it isn't really erase.It continues existing on your hard drive, even after you empty it from the Recycle Bin.This allows you (and other people) to Recover files you've deleted. If you're not careful, This will also allow other people to recover your confidential files, even if you think you've deleted them.
What Happen When You delete a file?
Windows and other operating system keep track of where files are on a hard drive through disk has a pointer that tells Windows where the file's data begins and ends. When you delete a file, Windows removes the pointer and marks the sectors as free. But that file is still recoverable by data recovery tools.
Why deleted file aren't erased Immediately?
If you're wondering why your computer doesn't just erase files when you delete the, it's actually pretty simple. Deleting a file's pointer and marking its space as available is an extremely fast operation. In contrast, actually erasing a file by overwriting its data takes significantly longer. For example, if you're deleting a 10 GB file, that would be near instantaneous. Actually to erase the file's content, it may take several minutes - just as long as if you were writing 10 gigabytes of data to your hard drive.
Preventing Deleted files from being recovered
You can use a utility that automatically wipes your hard drive's free space - by writing other data over free space on your hard drive, all deleted file will be erased.
For Example, CCleaner's integrated Drive Wiper tools can do this.
To make sure that a single file can't be recovered, you can use a "file-shredding" application such as Eraser to delete it. When a file is shredded or erased, not only is it deleted, but its data is overwritten entirely, preventing other people from recovering it.
To really prevent someone from recovering any of your data, you can use a disk-wiping program, such as DBAN (Darik's Boot and Nuke.) Burn DBAN to a CD, Boot from it, and it will erase everything from your hard drive, including your operating system and all your persanal files, overwriting them with useless data. This is very useful when getting rid of a computer - it helps you ensure all your personal data is erased.
Note : This article is only for educational purpose . I will not take any responsibility of misuse of this trick.