Is it true that deleted files still exist in my hardrives?

Derrik's Boot and Nuke is a great program you can run from a bootable CD or floppy...it will wipe out your entire machine and gives you the option to wipe it with the US Government's Department of Defense standard of wiping, where it puts just random 1s and 0s over and over again over your entire hard drive, making it hard (but not impossible) for someone to recover any information from it. You must have something highly illegal on your machine, and I don't care to know what it is. If it's nothing illegal, and you are just trying to hide a picture of some grown woman's **** from your wife, just delete it, empty trash bin, you'll be fine. No ordinary person will go through the trouble to actually recover that.