I remove the hard disk. Without the hard disk (or SSD solid state drive) I contend that the PC doesn't remember anything. OK, remove the keep-alive button cell battery as well.
I then open the hard disk assembly and do my best to heavily physically damage the recording surfaces. I seriously bend the disk so that I think it would take (expensive) serious effort to make it flat enough to be read again.
My goal is to make data recovery far more costly than the value of any data recovered.
My county has a computer and electronics recycling center. I think they are recycled for the precious metals they contain. The destroyed hard disk goes the same place. Your county may vary.
I've done about a dozen old PCs this way. Getting inside the disk drive assembly is the difficult part. I've seen photos of big "vending" machines which chew up disk drives, but I never expect to see one in person.
seacue