设置窗口区域的问题

Question

为什么我在Winme下，只能设置程序本身的窗口的区域，对其他程序的窗口进行SetWindowRgn会失败，但是在Win2000下却都可以？HWND hWnd = FindWindow(NULL,"Test");
if(hWnd!=NULL)
{
  HRGN hRgn = CreateEllipticRgn(0,0,200,100);
  if(hRgn==NULL)
    MessageBox(HWND_DESKTOP,"CreateEllipticRgn failed","Error",MB_ICONSTOP);
  else
  {
    if(!SetWindowRgn(hWnd,hRgn,TRUE))
      MessageBox(HWND_DESKTOP,"SetWindowRgn failed","Error",MB_ICONSTOP);
    DeleteObject(hRgn);
  }
}
else
  MessageBox(HWND_DESKTOP,"找不到窗口:Test","Error",MB_ICONWARNING);