For this guide I'll be primarily using File Shredder, which is another great open source utility (provided for free under the GNU public license). File Shredder, along with this guide, are both meant to help you destroy files - not an entire hard drive full of them. If you'd like to completely obliterate everything on the hard drive, take a look at DBAN (Darik's Boot and Nuke). Just be careful with it -- it can cause damage if you don't know what you're doing (perhaps a future guide from us!).
Set Phasers to Kill
Now, the first logical step is to try and shove your hard drive's platter through the shredder. This didn't turn out very well for my shredder, so I wouldn't recommend it.
Go grab File Shredder here (use the red download button at the bottom of the page, not the ad button). Launch it once it's done! Here's what you should see:
Pretty simple, right? The interface is actually very easy to use and gets the job done nicely. Before you go shred-happy, we need to check the settings on File Shredder and set them to be specific to your task. Click on "Shredder Settings" on the left side. The "Program Settings" tab should have "Enable Shell Integration," "When Shred Files," and "When adding or removing" checked -- leave them that way.
Go to the "Algorithms" tab, Captain.
The drop-down has five options (at time of writing), ranging from "eh, I don't need it anymore" to "GET IT OFF! GET IT OFF!" in terms of destructiveness. Be careful what you wish for: Using some of the more thorough settings will make file retrieval almost impossible for a non-lab setting. With some hard work, I was able to partially recover files that were shredded with the "simple one pass" option, so if you want to be absolutely certain no one will ever see that file, ever, not even in 200 years when aliens are excavating your home, use the most thorough two options.
These will take significantly longer, so keep that in mind.
Now it's time to decide what you want to delete. For demonstration purposes, I've created a volatile password and configuration file, along with a series of subdirectories (which are composed of images, docs, excel sheets, etc.). We can delete both individual files and entire directories (recursively, if desired). Start with a test file so you get the feel for it.
Click 'add file' or 'add folder' to get started.
Double-check, triple-check, quadruple-check: Do you REALLY want to delete these? Got it. Hit that "Shred Files" button and watch 'em disappear.
That's it! Go wild (but be careful, damn it!). Make sure you've backed up important stuff on secured and encrypted drives. Damn, that's another guide to add to the list.
Comment if you need help. See ya' next time.