There's no reason to set TS in cr0 when returning from a w32 callback - either the thread will have used the FPU, in this case we don't do anything, or it has not used the FPU and we copy it's saved FPU state (if any) into the original's stack FPU area (in this case TS in cr0 will already be set and we don't need to set it)

reactos/ntoskrnl/ps

diff -u -r1.20 -r1.21
--- w32call.c	27 Nov 2004 23:50:27 -0000	1.20
+++ w32call.c	28 Nov 2004 18:14:02 -0000	1.21
@@ -1,4 +1,4 @@

-/* $Id: w32call.c,v 1.20 2004/11/27 23:50:27 hbirr Exp $

+/* $Id: w32call.c,v 1.21 2004/11/28 18:14:02 blight Exp $

  *
  * COPYRIGHT:              See COPYING in the top level directory
  * PROJECT:                ReactOS kernel

@@ -140,7 +140,6 @@

   Thread->Tcb.TrapFrame = SavedTrapFrame;
   Thread->Tcb.CallbackStack = SavedCallbackStack;
   KeGetCurrentKPCR()->TSS->Esp0 = (ULONG)SavedExceptionStack;

-  Ke386SetCr0(Ke386GetCr0() | X86_CR0_TS); /* set TS */

   KeStackSwitchAndRet((PVOID)(OldStack + 1));
 
   /* Should never return. */