'zeroing' every sector is probably 'good enough'.

if the laptops are functional:

boot a target laptop to a windows install disk or usb. at the first prompt, hit SHIFT-F10 to open a command window.

diskpart

list disk

select disk 0
(laptop internal drive should be disk 0)

clean all
(this will take time. on an older laptop, up to maybe an hour per 250gb of hdd capacity)

exit

power off.

if they're NOT functional laptops:

remove the hdd (search the make and model at yt for guides), connect to another pc (via usb adapter) or internal inside a dt. boot to that system's normal windows, run diskpart from an admin command prompt.. do the same as above but note that in this case: IT WON'T BE DISK 0 -- be sure to 'select' the correct disk