There are a number of programs available that will fill your disk with zeros whenever a file etc is erased.
they are used extensively by various organisations and are readily available even free versions.
this is to help you if you want to get rid of an old computer.
if you want to just rid an old hard drive, you can always physically damage it if you are worried.
It costs far too much to obtain info from a damaged disk, tto make it worthwhile for anyone to be interested. apart from governmental sources, or company espionage.