Bank account details and personal information of potentially thousands of Britons are being sold in West Africa for less than £20 each - according to a new survey.
http://news.bbc.co.uk/1/hi/programmes/real_story/4791167.stm
So be extra careful with that redundant hard drive!
It would be best to remove the hard drive, destroy it completely, and then get rid of the old computer in whatever manner is available. If selling it and the buyer does not want it without a hard drive, those are relatively cheap these days. So, install a new one and then sell the old computer for a price that includes the cost of the new hard drive.
You can also use a program like Eraser
http://www.heidi.ie/eraser/download.php
to securely delete files and clear the blank spaces on your hard drive.
Let it run over night - it takes a long time even with just 3 randomizing passes.