diff --git a/win32ss/include/ntuser.h b/win32ss/include/ntuser.h
index 0eea35fffa4..68999e7f932 100644
--- a/win32ss/include/ntuser.h
+++ b/win32ss/include/ntuser.h
@@ -1561,9 +1561,9 @@ NtUserBuildHwndList(
     HWND hwndParent,
     BOOLEAN bChildren,
     ULONG dwThreadId,
-    ULONG lParam,
-    HWND *pWnd,
-    ULONG *pBufSize);
+    ULONG cHwnd,
+    HWND *phwndList,
+    ULONG *pcHwndNeeded);
 
 NTSTATUS
 NTAPI
diff --git a/win32ss/user/ntuser/window.c b/win32ss/user/ntuser/window.c
index cc34e5e6256..5f588406bdd 100644
--- a/win32ss/user/ntuser/window.c
+++ b/win32ss/user/ntuser/window.c
@@ -1500,22 +1500,24 @@ VOID FASTCALL IntFreeHwndList(PWINDOWLIST pwlTarget)
  * @implemented
  */
 NTSTATUS
-APIENTRY
+NTAPI
 NtUserBuildHwndList(
    HDESK hDesktop,
    HWND hwndParent,
    BOOLEAN bChildren,
    ULONG dwThreadId,
-   ULONG lParam,
-   HWND* pWnd,
-   ULONG* pBufSize)
+   ULONG cHwnd,
+   HWND* phwndList,
+   ULONG* pcHwndNeeded)
 {
    NTSTATUS Status;
    ULONG dwCount = 0;
 
-   if (pBufSize == 0)
+   if (pcHwndNeeded == NULL)
        return ERROR_INVALID_PARAMETER;
 
+   UserEnterExclusive();
+
    if (hwndParent || !dwThreadId)
    {
       PDESKTOP Desktop;
@@ -1525,7 +1527,8 @@ NtUserBuildHwndList(
       {
          if(hDesktop == NULL && !(Desktop = IntGetActiveDesktop()))
          {
-            return ERROR_INVALID_HANDLE;
+            Status = ERROR_INVALID_HANDLE;
+            goto Quit;
          }
 
          if(hDesktop)
@@ -1536,7 +1539,8 @@ NtUserBuildHwndList(
                                               &Desktop);
             if(!NT_SUCCESS(Status))
             {
-               return ERROR_INVALID_HANDLE;
+                Status = ERROR_INVALID_HANDLE;
+                goto Quit;
             }
          }
          hwndParent = Desktop->DesktopWindow;
@@ -1556,13 +1560,13 @@ NtUserBuildHwndList(
          {
             if (bGoDown)
             {
-               if(dwCount++ < *pBufSize && pWnd)
+               if (dwCount++ < cHwnd && phwndList)
                {
                   _SEH2_TRY
                   {
-                     ProbeForWrite(pWnd, sizeof(HWND), 1);
-                     *pWnd = Window->head.h;
-                     pWnd++;
+                     ProbeForWrite(phwndList, sizeof(HWND), 1);
+                     *phwndList = Window->head.h;
+                     phwndList++;
                   }
                   _SEH2_EXCEPT(EXCEPTION_EXECUTE_HANDLER)
                   {
@@ -1571,7 +1575,6 @@ NtUserBuildHwndList(
                   _SEH2_END
                   if(!NT_SUCCESS(Status))
                   {
-                     SetLastNtError(Status);
                      break;
                   }
                }
@@ -1612,13 +1615,15 @@ NtUserBuildHwndList(
       if (!NT_SUCCESS(Status))
       {
          ERR("Thread Id is not valid!\n");
-         return ERROR_INVALID_PARAMETER;
+         Status = ERROR_INVALID_PARAMETER;
+         goto Quit;
       }
       if (!(W32Thread = (PTHREADINFO)Thread->Tcb.Win32Thread))
       {
          ObDereferenceObject(Thread);
          TRACE("Tried to enumerate windows of a non gui thread\n");
-         return ERROR_INVALID_PARAMETER;
+         Status = ERROR_INVALID_PARAMETER;
+         goto Quit;
       }
 
      // Do not use Thread link list due to co_UserFreeWindow!!!
@@ -1633,13 +1638,13 @@ NtUserBuildHwndList(
             Window = ValidateHwndNoErr(List[i]);
             if (Window && Window->head.pti == W32Thread)
             {
-               if (dwCount < *pBufSize && pWnd)
+               if (dwCount < cHwnd && phwndList)
                {
                   _SEH2_TRY
                   {
-                     ProbeForWrite(pWnd, sizeof(HWND), 1);
-                     *pWnd = Window->head.h;
-                     pWnd++;
+                     ProbeForWrite(phwndList, sizeof(HWND), 1);
+                     *phwndList = Window->head.h;
+                     phwndList++;
                   }
                   _SEH2_EXCEPT(EXCEPTION_EXECUTE_HANDLER)
                   {
@@ -1649,7 +1654,6 @@ NtUserBuildHwndList(
                   if (!NT_SUCCESS(Status))
                   {
                      ERR("Failure to build window list!\n");
-                     SetLastNtError(Status);
                      break;
                   }
                }
@@ -1662,8 +1666,13 @@ NtUserBuildHwndList(
       ObDereferenceObject(Thread);
    }
 
-   *pBufSize = dwCount;
-   return STATUS_SUCCESS;
+   *pcHwndNeeded = dwCount;
+   Status = STATUS_SUCCESS;
+
+Quit:
+   SetLastNtError(Status);
+   UserLeave();
+   return Status;
 }
 
 static void IntSendParentNotify( PWND pWindow, UINT msg )
diff --git a/win32ss/user/user32/windows/mdi.c b/win32ss/user/user32/windows/mdi.c
index 59a3d06c506..9424b4cfe6e 100644
--- a/win32ss/user/user32/windows/mdi.c
+++ b/win32ss/user/user32/windows/mdi.c
@@ -145,7 +145,7 @@ HWND* WIN_ListChildren (HWND hWndparent)
   HANDLE hHeap;
   NTSTATUS Status;
 
-  Status = NtUserBuildHwndList ( NULL, hWndparent, FALSE, 0, 0, NULL, &dwCount );
+  Status = NtUserBuildHwndList(NULL, hWndparent, FALSE, 0, dwCount, NULL, &dwCount);
 
   if ( !NT_SUCCESS( Status ) )
     return 0;
@@ -161,7 +161,7 @@ HWND* WIN_ListChildren (HWND hWndparent)
     }
 
   /* now call kernel again to fill the buffer this time */
-  Status = NtUserBuildHwndList (NULL, hWndparent, FALSE, 0, 0, pHwnd, &dwCount );
+  Status = NtUserBuildHwndList(NULL, hWndparent, FALSE, 0, dwCount, pHwnd, &dwCount);
 
   if ( !NT_SUCCESS( Status ) )
     {
diff --git a/win32ss/user/user32/windows/window.c b/win32ss/user/user32/windows/window.c
index 0946c5b68ca..93dd53358c5 100644
--- a/win32ss/user/user32/windows/window.c
+++ b/win32ss/user/user32/windows/window.c
@@ -701,7 +701,7 @@ User32EnumWindows(HDESK hDesktop,
                                  hWndparent,
                                  bChildren,
                                  dwThreadId,
-                                 lParam,
+                                 dwCount,
                                  NULL,
                                  &dwCount);
     if (!NT_SUCCESS(Status))
@@ -729,7 +729,7 @@ User32EnumWindows(HDESK hDesktop,
                                  hWndparent,
                                  bChildren,
                                  dwThreadId,
-                                 lParam,
+                                 dwCount,
                                  pHwnd,
                                  &dwCount);
     if (!NT_SUCCESS(Status))