Is it possible to perform a factory reset on an old Apple iMac (OS X Snow Leopard) without a disk My mum's got an old Apple iMac from ca. 2009 which after 10 years is pretty much for it.

I'd like to wipe the computer to get rid of any personal data before we get rid of it. Just in case.

As an older machine, it's still running OS X Snow Leopard.

Unfortunately I don't have any of the install disks, because, well, I knew Mum was gonna get rid of the computer so I binned them when I was having a big sort out when I moved house. Call me stupid but I didn't exactly expect to need the install disk to uninstall it. But then I'd forgotten the old adage, if you want to lock yourself out of your house, make sure you keep your house keys with you at all times.
Erasing an old 'spinning rust' Hard Drive is not the safest or most secure way to totally prevent data recovery by a bad actor, even if you use the old 'secure erase' functions.

The simplest &amp; most secure method is to destroy it.
Whether that's by drilling holes through it or taking a very large hammer to it is pretty much dependant on how much fun you may find either option ;)

The remains are recyclable no matter which way you do it.