XP: an ancient 14 year old OS that doesn't support multi core CPUs, moving on...
It doesn't properly support them. The scheduler differences.
some pretty misguided information here
XP supports multiple cores just fine, its number of CPUs that is what people get confused about, and would have helped if it had been described as number of CPU sockets that the OS supports...i.e. XP Home supports 1 x CPU socket systems while XP Pro could support 2 x CPU socket systems
IIRC, 32bit XP supports up 32 cores/threads divided up between either 1 or 2 processors (ie either 1 x 32 core/thread CPU or 2 x 16 core/thread CPU) while XP Pro x64 supports up to 64 cores, same deal. So with 32bit XP Pro you could run a dual 8-core Xeons with HT and have support for all 32 processing threads.
edit, yup:
https://en.wikipedia.org/wiki/Windows_XP#Processor_limits
that being said, just because it can support multiple cores doesn't mean you would want to use XP with modern hardware (particularly under 32bit with the 4GB limitation, even more so if you're using a video card with more than 512MB of ram)