求助模拟ctrl+alt+del

想模拟ctrl+alt+del弹出任务管理器，使用keybd_event但是无效，模拟ctrl+shift+esc却能够成功弹出，请教原因 keybd_event(VK_LSHIFT,0,0,0);
keybd_event(VK_LCONTROL,0,0,0); keybd_event(VK_ESCAPE,0,0,0);
keybd_event(VK_ESCAPE,0,KEYEVENTF_KEYUP,0);         keybd_event(VK_LSHIFT,0,KEYEVENTF_KEYUP,0);
keybd_event(VK_LCONTROL,0,KEYEVENTF_KEYUP,0);这样模拟ctrl+shift+esc可以成功
但相应的换成 
         keybd_event(VK_LMENU,0,0,0);
keybd_event(VK_LCONTROL,0,0,0); keybd_event(VK_DELETE,0,0,0);
keybd_event(VK_DELETE,0,KEYEVENTF_KEYUP,0);         keybd_event(VK_LMENU,0,KEYEVENTF_KEYUP,0);
keybd_event(VK_LCONTROL,0,KEYEVENTF_KEYUP,0); 却无法模拟 ctrl+alt+del
请指教，谢谢！