[userbinator]

Almost everybody on XP was using a 32-bit version, which only uses two cores at the most.

Where did that misinformation come from? In the late 2000s, quad-core Q6600s were very common at the time and they certainly had all 4 cores active under 32-bit XP.

[toast0]

I think there's a misunderstanding about the processor limits. I can't find an official doc because everyone links to a KB article that must have showed them in the past, but now talks about virtual memory...

But XP docs talk about a limit on the number of processors that is a limit on the number of sockets. If you've got an old quad pentium 2 system, with four slotted processors, XP Pro is only going to run on two of them, because it has a 2 processor limit. But multiple cores/threads are subject to a different limit that I can't find. Home may have been single socket?

Some people might have tried to run it in virtualization, and not been careful to setup virtualized cpus as cores in a single socket rather than multiple sockets.