Sensitive digital data must be securely erased, once a computer system is marked for disposal!
I use the Eraser software application to securely remove data from my systems. I run Eraser at least once a week, to prevent deleted (including temporary) files from being reconstructed, and I use it or PGP Wipe, which came with PGP Desktop Home to ensure that the disks I am donating to charity or giving to someone else cannot be used to reconstruct any of my data.
Every computer that I have helped set up for a charity that was donated from a commercial corporation contained sensitive data files. Some were deleted and easily undeleted, others were still in native form, ready to be accessed. Do not allow data to leave your control: encrypt it while it is under your control and securely delete it before relinquishing control of the computer.