source: http://tech.yahoo.com/blogs/null/50840

When you delete a file from your computer (and it needn't be Windows, this is common to every type of PC), that file doesn't "go away." Rather, to save wear and tear on your hard drive and to simplify the operation, your computer just eliminates the record of where the file began. Think of your PC as containing a giant "shopping list" of all the files on its hard drive. Delete the canned peaches off that shopping list and the store doesn't actually get rid of the peaches. It just "forgets" that they are there. The space allocated to the peaches remains there until the store needs the space for something else.

There's good and bad in this. The good is that if you accidentally delete something you have a good chance of being able to get it back. The bad: So can anyone else.

These deleted files aren't accessible via Windows, but data recovery software like File Scavenger can quickly recover most recently-deleted data from your PC as if it had never been deleted at all. If you're sure you want to delete those files for good so programs like this won't work, there's plenty of software for that too. I often recommend BCWipe, which lets you permanently wipe only the free space on your hard drive while leaving the non-deleted files intact by overwriting those deleted files with lots of random data so it can't be recovered. Other software like Kill Disk does the same thing to the entire hard drive, rather than just the blank space, leaving an entirely empty, unrecoverable disk when you're done

While BCWipe is something you run periodically, there's also software to let you do this on the fly, essentially replacing the recycle bin with the equivalent of a paper shredder that wipes any file you delete for good. If you install one, tread with caution: Once you delete something, even by mistake, there's no going back. Simple File Shredder is a good (and free) choice to check out.

If you're accident-prone, there's software that goes the other way too, putting extra safeguards on deleted files and making it easier to recover mistakenly deleted data. Check out Norton SystemWorks, which includes a "Protected Recycle Bin."