Yep, in addition to the DOD-type file shredders already suggested (and @quietman's other excellent advice), the "most secure" options involve the use of a screwdriver (and perhaps a hammer):
- Physically remove the hard drive from the case and take it to a bonded, secure shredding service (most charge only a nominal fee and you can watch them do it); OR
- Physically remove the hard drive from the case and then use the hammer (or a sledge hammer) to do the deed yourself.
Removing the hard drive does lessen the "value" of the computer, if you plan to donate it.
But, if you plan to dispose of it (hopefully via an environmentally responsible service), then it won't matter.
And you can sleep well, knowing that anything on that hard drive is permanently gone gone gone...