Well, you can try a free (or paid for) real deletion utility if you like, and whether or how much you trust it is up to you, but if you're really serious about making sure that nobody can ever read the information again there's always the same way of ensuring it that you apply to hard disks when decomissioning a sensitive machine - take a hammer to it.
As ever, you start by deciding on the threat model you wish to address, and then work out what's a proportionate response.