First of all, Use Darik's Boot & Nuke (Google it).
After that, remove the HDD, and build a simple DIY Demagnetizer
Shop built demagnetizer
Make sure that your drive was degaussed. For that, Connect it back to the PC, and turn it on. It should do a rattling sound (clac clac clac, etc) when the head tries to move to the first magnetick track and can't find it, returns to it's housing, seeks track number zero again, and again, etc. forever. At that point, not even a low level formatting will allow any of the leftover data to be recovered by conventional (without very specialized expensive laboratory equipment, clean room (zero dust), etc) means.
Dissasemble the drive. You can get rid of the carcass, head, and boards right away.
Take the plate (or plates) and with patience, drill as many holes in it as you can. Collect all of the shavings and dust. You should by that point have a plate that is more holes than plate.
After that, take all of the shavings, dust, and plate remnants, and put them on a steel or ceramic container. Use a blowtorch to heat all of the remaining elements over the Curie point (Around 700/800 ºc). At that point, all material loses its magnetism. Then, as it cools down, all of it realigns with the magnetic field of Earth.
Once it's cool, you can throw it away.
This method that i've described is used by intelligence organizations from all the world. If that's good enough for them....