How to prepare old PCs for disposal?

We have a truckload of old electronic equipment that will be picked up for recycling the week of January 21.

We need to erase/remove/destroy the data on all PC hard drives prior to disposal. The data is not necessarily private or confidential, but at the same time it should not be avilable to the public.

Most of the PCs are Windows 3.1 (some only have DOS and a couple machines have Windows 95).

What would be the most effective way to accomplish this task?

Thank you in advance for any suggestions!