Physical destruction is much the quickest, surest and most satisfying method. Even if you want to pass on the computer for re-use, fitting a new solid state disk is cheap and easy, and will make the computer run faster than ever before. But if you REALLY MUST keep the old hard disk, there is a choice of several disk wipe utilities:

Ccleaner will wipe a hard disk, but not the one you're booted from. You can connect your old hard disk to another computer with a USB-SATA adapter in order to wipe it.

You can make a bootable memory stick or CD containing DBAN, which will wipe your hard disk.

Both the above may leave fragments of data in remapped and hidden disk sectors. Blancco (from the same source as DBAN) is the professional disk wipe utility, carrying various government and industry standard certifications and approvals. An evaluation licence is available.