Consumer magazine Which? advises that hard disk drives should be smashed into pieces using a hammer if you no longer use them. This solution, although quick, is neither easy nor painless especially if you will be opening a computer for the first time.
Destroying a hard disk drive will leave you with a near-useless personal computer with little reselling value. If you want to get rid of a computer all together, we would suggest that you actually donate the computer to a charity or to one of your relatives/friends.
If you plan on selling the computer on Ebay, we would recommend DBAN (Darik's Boot And Nuke) to "clean" your computer. You can make a bootable CD or DVD using an ISO file. It will automatically and completely delete the contents of any hard disk drive it will detect.
This free product is one of many which allow users to clean their hard disk drive. Other noteworthy applications include Heidi Eraser available from Heidi Computers and Lenovo's own Secure Data Displosal 1.3.
Another technique involves filling up your hard disk drive with files. For example, you could download an ISO copy of any Linux distribution and make copies of them (if you are in Windows), just to fill space. This will overwrite data the hard drive until it is full. Then format it.
Alternatively, you can take all your data, compress and encrypt using a strong encryption and then format the hard disk drive. Even if criminals manage to recover the encrypted file, trying to decrypt the recovered file will be a very, very difficult task.
Finally, rather than smashing it, while not recycle your hard disk drive, either as an external hard disk drive (very handy), inside a file/media server or as a catchy door stopper.
PCs - Recycled / Special Charity Pricing @ (IT For Charities)
Secure Data Disposal 1.3 @ (Lenovo)
Data Sanitization Utilities @ (Computer Power User)
Eraser @ (Heidi)