XP comes in a 64bit version?
It's not as well known as most other versions of Windows XP and Vista.
I believe it was designed more for workstation use than home users and was marketed as such, but with RAM getting much cheaper and especially since Vista a lot of people have migrated to it.
I'm hardcore, I was using it before Vista on my laptop, just because I could.
I'll also repeat what I've said before - on three entirely different computers with lots of different peripherals, I have yet to encounter a single driver issue, disregarding an absolutely ancient scanner that drivers for any OS were all but impossible to find. Software issues were also limited to OS-limited programs, like XP Powertoys, some programs that integrate directly into Windows components such as explorer, and some older games. Just like Vista itself (trying to tie this in vaguely with the actual topic of this thread) compatibility is nowhere near as bad as a lot of people make out.