I have ~37 khash/s under Windows  XP x64 and ~53 khash/s under Windows 8 x64.
8 threads, AMD FX 8320.
minerd 2.23 x64
Possibly this is due to difference in scheduler policy: Windows 8 assigns dedicated kernel for each thread whereas XP kicks threads from one kernel to another.

I do not want to upgrade Windows XP to Windows 8. Instead, I would like to change Windows XP scheduler policy - is it possible?