While I wouldn't go so far as to call it "foolproof," I'm sticking with '95.
About a month ago I was asking my son what type of computer and operating system he has in work. He is rising in a well known company and I figured he had a super duper setup, since his computer at home is state of the art.
When he told me about his modest work computer with Win 95 I was amazed. He explaind his company regularly upgrades the computers and the Operating Systems. He has seen plenty of new upgrades to other Win systems that are less reliable. He passes on the upgrades in work and just keeps chugging along on an older computer with win 95 .
The reason his home computer is so far advanced over his work computer is they refused to build the older systems with win 95 for his home use.
So now he has at home, a 2 GHZ 512 super duper Ram -XP operating system with a major security leak. - Tom