When you delete a file, it doesn't actually go away--even after you've emptied the Recycle Bin. The actual bits remain written on the drive until some other disk activity writes over them. Even when you format a drive, the files are still there for those who want and know how to read them.
If you want to truly and securely delete a file, or the contents of an entire drive, you need software that will overwrite the space where the file(s) once sat. Fortunately, several free programs can do this.
First, I recommend Eraser, which integrates with Windows Explorer. Once it's installed, you can just right-click a file or folder and select Eraser. There's even an option to erase the file the next time you boot--handy if Windows won't let you erase it now.
Another option: Delete the files the conventional way, empty the recycle bin, then use CCleaner to overwrite your drive's free space. This extremely useful tool can do all sorts of Windows scrubbing chores. You'll find CCleaner's Drive Wiper tool in the Tools tab.
Both of these programs offer various wiping techniques that overwrite the drive space multiple times. The implication, of course, is that overwriting a file 35 times is more secure than overwriting it only once.
But according to Russell Chozick of Flashback Data (a company that that does forensic data recovery for law enforcement organizations as well as conventional data recovery), one pass is enough with today's drives. Chozick told me that it "used to be possible to see what was overwritten, but now data is too dense, [making] a single-pass overwrite sufficient."
Something else to think about: If you have sensitive files that you'll eventually want to securely delete, you should encrypt them now. I recommend the free, open-source Truecrypt.
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