We know the score. We're sophisticated users. We know that when you delete a file on Windows the only thing that get's erased is the pointer to the file. The disk blocks are returned to the free blocks pool and if they get re-written with actual data then the file is really deleted.
Just for fun I ran a deleted file recovery product on my month-old PC - Recover My Files. After it found more than 62,000 deleted files I cancelled its execution. It was in heaven. Very few of the deleted files had actually been over-written and most were completely recoverable.
So then I Google'd 'secure delete' and came across many software products that I could buy for less than $30/£20 that would actually over-write files to be deleted and, in some cases, clean an entire hard drive.
And the point is...? Just to remind us how vulnerable we are when we discard old PCs or let hackers in to our systems because of inadequate or turned-off firewalls. Dammed resourceful these Nigerian scamsters. Take care.