It's long been understood that if you buy a used PC, it's likley that you will be able to recover all kinds of juicy secrets from its hard drive (I've had this experience both in used PCs I've bought in in hard-drives I've dumpster-dived). Now a group of researchers have done a formal study, discovering financial info, personal stuff on children and even good blackmail stuff on hard-drives bought at auction and in flea markets.
A research team from Glamorgan University analysed 111 supposedly clean hard drives, bought for less than £1,000, and found that more than half still contained personal information. This included national insurance numbers, evidence of a married woman's affair and detailed biographical information about children.
Ninety-seven of the hard drives were bought on eBay and four at car boot sales. As a control experiment, ten drives were also sourced from LCS Remploy, a company specialising in the destruction of data. All proved to be clean.
The original owners of the other 101 drives included universities, multinational companies and a Church of England primary school in East Yorkshire, all of which were breaking the Data Protection Act by failing to dispose of the information effectively.
(via Schneier)
Update: Stef sez, "Boing Boing readers may benefit from the free & open-source (GPL) application Darik's Boot & Nuke: it's a self-contained bootable floppy/CD that securely completely wipes the hard disks of most computers. There's even a Mac version now :) Works very well, I use it here at work to make sure that the drives of all the old machines we give away are securely wiped."