As has been said here, deleting files just marks the space as available for use again. It's like putting a piece of paper you've written on onto a "recycle" pile - the stuff on it deosn't get rubbed out until someone comes to write new stuff on it - why would you bother? That's why it's so easy to get stuff back - you just have to know where to look. "Quick" formatting, similarly, doesn't erase what's there, it just changes the index of the book to say all the pages are blank.
Writing over the whole disk just once will stop anyone getting your data without specialist hardware, by which I mean a big machine they put the innards of your hard drive into. Basically only the feds have these, so no worries if you have nothing illegal to hide. C cleaner does nothing like this as far as I'm aware, but BCWipe is a very good program. If you really want your data to be safe for whatever reason, don't even consider selling the disk, thermite is the only way, seriously.
Going slightly off topic, "hackers" like in die hard 4.0 are nonsense, and wiki the difference between "hacker" and "cracker". Snort's a good tool, but of course these days everyone expects it and tip-toes round it.
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