Strange Behavior in NT 4.0

I have a Win32 C app that works fine in W95 and NT 3.51. In NT 4.0, however, the client area of the app window (it's SDI) does not seem to get painted properly. Instead of a white background, it shows whatever was on the screen behind it when the app was launched. When the screen refreshes, the client area displays again what is on the screen, not the usual white background. (In all other respects, the app functions normally.)

I handle the WM_PAINT message by calling this function:

void RepaintScreen(HWND hWnd, PAINTSTRUCT ps, short ylen)      {
      
int iCtr;
HDC hDc;
int begin = 0;



          Y = 2;  
          if ( SCREEN_LINES > (ylen/Spacing) && bScreenLines )
                begin = SCREEN_LINES - (ylen/Spacing);
          //hDc = BeginPaint(hWnd,&ps);
          hDc = ps.hdc;
             for (iCtr = begin; iCtr < SCREEN_LINES; iCtr++)      
             {
                   if ( lpsScrn[iCtr][0] != '\0' )
                   {
                        TextOut(hDc, 1, Y, lpsScrn[iCtr], lstrlen(lpsScrn[iCtr]));                     
                      Y = CheckForScroll(hWnd,
                                                Y,
                                                Spacing,
                                                lstrlen(lpsScrn[iCtr]) * tm.tmMaxCharWidth);
                   }
            }
              EndPaint(hWnd,&ps);
         
               return;
}


RepaintScreen() calls CheckForScroll():

int CheckForScroll(hWnd,CurrentPosition,Spacing, Length)
HWND hWnd;
int CurrentPosition;
int Spacing;
int Length;
{
    RECT rect;                        /* RECT structure for validation */
    rect.top = CurrentPosition;       /* top of last line of text       */
    rect.bottom = CurrentPosition+Spacing+1; /* bottom of last line       */
    rect.left = 1;                  /* left most column of line       */
    rect.right = Length+1;            /* right most column of line       */
            
    GetClientRect(hWnd,(LPRECT)&rect);      /* get rect for current client   */
    if(CurrentPosition + (Spacing*2) > rect.bottom) /* will line fit       */
    {
                              /* if not scroll window and        */
                              /* update client window             */
            ScrollWindowEx(hWnd,0,-(Spacing+1), (CONST RECT*) NULL,
                              (CONST RECT*) NULL,
                              (HRGN)NULL,(RECT FAR*)NULL, 0);
      
            return(CurrentPosition);
    }
    return(CurrentPosition+Spacing);
}