When we think of shredders, most of us probably think of paper shredders — but in this digital age, another kind of shredder is important, too: a computer program that "shreds" — that is, securely deletes — secret files on your hard drive.
Normally then you erase a file from your computer (by dragging it to the Recycle Bin or Trash Can, then emptying it) the space that the file occupies is released back to the system as "free space", but until it gets physically rewritten over the data still exists in whole or in part. There are utilities, not too hard to use, that can read such areas on the drive - and if you know enough you can find them in your normal PC operating system. So someone can dig through your digital trash and recover the files that you thought were deleted.
Therefore, if the concern is to make sure that a deleted file is actually once-and-for-all gone, you may have to take rather extraordinary steps.
It's much harder to wipe data on any purely magnetic medium beyond the possibility of recovery. Writing over a file leaves traces of the original characters there which immensely sensitive equipment can detect; as narrow as the read-write tracks are on these devices, when we're thinking at the atomic level there is plenty of space to spare and the erasing process does not necessarily cover the entire width of the track. Some white papers recommend erasing the file SEVEN TIMES to be sure (and the US Government standard specifies this). An even more drastic procedure is to de-gauss the device: that will utterly wipe out all the magnetic fields on the drive and require that a factory format be applied if the device is to be usable again. There is no possibility of anything surviving that much diligence!
But there are many resources available that offer a very high probability of wearing the data out of existence without going as far as de-gaussing. One such is "Advanced File Shredder" from Ashkon Systems. It follows the 7-pass standard, uses the Guttman method of file shredding (see the site: www.ashkon.com/shredder.html), rewrites all the disk sector, track, and cluster information, and as a bonus wipes out internet "temp" files and cookies (using its own erasure methods, to be sure!). There are many others available for the searching.
It is also worth looking at programs that clear all internet fiddling with your hard drives. Even with the greatest diligence, and with the most innocent programs, gads of data are stored all over your drives which you aren't told about and which may cause actions that you don't want. One popular free file shredder program, which can be downloaded as freeware, is Trend Micro's CWShredder, which is designed to "shred" or remove all traces of a particular browser hijacker that can alter users' bookmarks and desktop and dramatically slow down and otherwise mess up a PC. The search for "pc file shredders" will bring up "history erasers" for this purpose as well as the file erasers that are the subject of this article.