I know it is not a perfect solution. Malware-types can have access to a pc assembly line, and they sometimes could be found working in famous software making companies, but my practise has long been, when buying a new or used pc, to reformat it. I suppose I'd be alright, as long as I didn't "reintroduce" the malware, unwittingly, during the reformat! I've even had av programs scanned for viruses and malware.
Some of you will remember the old days when netbus and sub7 were the big news (in the 1990's). Unhappily, in the late 1990's, I was "netbused." It was an eye-opener, as I just could not believe any pc rogue carred about my pc. I had to learn better...Sigh.