Downloads the deed to my house?!
I think there is a fundamental misunderstanding about what old systems are used for and what is stored on them. I don't think it's smart to keep anything sensitive on your main rig let alone your old one. I keep it on an external drive connected to a power surge strip with an ON/OFF switch, which is OFF 99% of the time. Closest thing to not ever using it, ever.
But for the sake of argument, I do have identical anti virus software installed on both Windows 8 and Windows XP. They both detect the same malware when exposed to it. They both lite up like a X-Mas tree when the same malware sites are visited using the same Firefox browser...
I have Windows 8 protected with what I tested to be one of the best post zero-day anti-virus definitions (Avira freeware) in combo with best Firewall I could find being tested out there (Comodo Firewall Freeware) and despite of all that
I am still certain, that a high level computer hacker can get access to my system from anywhere in the world at any time they want.
I am only protected against script kiddies and inexperienced copy-paste hackers who download attack software from underground web sites.
I am protected from them on my Windows 8 through the same third-party software I mentioned that is also installed on my old Windows XP systems.
We are simply not going to update numerous old machines because
they are not our main rigs... and we are not going to throw them away either, they are perfectly fine for what they are being used for. I am not sure what the end goal of the argument is, I can't even install Windows 8 on my old machines in the house, but they run Windows 2000/Windows XP dual boots just fine.