This morning's London's Metro had a small article about how bank details extracted from old computers were being sold to Nigerians.
I suspect that they buy computer lots from Ebay and then use widely-available tools and uneease hard disk drives. The investigators have literally found thousands of computers in markets of Lagos, with a sizeable amount coming from UK. It is well known that third world countries are a fertile market for second hand computers, which are often out-of-lease office machines that have been replaced. Hence, one can expect some of them to contain sensitive information like credit card numbers, account details and passwords
The Open Source Movement has a number of tools available at hand. The most popular is Darik's Boot and Nuke which can be activated from a boot floppy, from a CDROM or from one of those ubiquitous USB drives. It is free and works reasonably well. There's even an active and dedicated support forum here.
You can also try Heidi's Eraser which is also free and whose source code is GPLed. The application got a best file shredder award from leading German publication Chiponline.de.
Note that both solutions described above are not failible. Given (lot of) time and (lots of) money, it would be possible to recover from the drives. But I guess that Nigerian fraudsters have none of them in great quantities.