The Athlon x4 860k uses the FM2+ socket, and the motherboard determines what OS can be used.
Older motherboards like this can use windows XP 32bit.
AMD Athlon X4 860K, this processor itself is compatible with Microsoft Windows XP and Windows XP Professional x64. But it depends on the motherboard manufacturers who provide firmware to make support or not. In order to run Windows XP, the system should provide support for CSM (Compatibility Support Module) and proper ACPI support for legacy system. I experienced many systems which provides the CSM, but does not maintain the compatibilities towards legacy OS, it would fail when booting up from the installation media, with BSOD and saying something wrong with file ACPI.sys.
As you know, Windows XP and XP x64 do not support message passing based interrupts, if firmware without providing emulation or backward compatibilities, there would no way to get around that problem. Or in other words, exactly, whether compatible with Windows XP does not lie on the processor, but the system platform. So you'd better contact with motherboard vendors before purchasing. OK, I just wish you good luck, and have a good journey to re-visit the world of Windows XP!