Can the Windows XP built-in Bluetooth stack support HSP (headset profile) as an extra audio device, without third-party drivers?
Windows XP supports - PAN, SPP, DUN, HID, HCRP out of the box... Looking in the support documentation, but can't find support for headsets.
You could always use an alternative Bluetooth stack at your own risk.
(From Windows 7 guide above)
So, if Windows 7 doesn't support it, I am pretty sure XP won't