When you log into a Windows server via the Remote Desktop Protocol (RDP) in some cases you need to send the Alt-Control-Delete ( in other notations Alt-Ctrl-Del, Control-Alt-Delete, Ctrl-Alt-Del ) key combination to the remote server.
From a Macintosh laptop
On a Macintosh laptop press the fn+control+option+delete keys to send Alt-Control-Delete to the server.
From a Windows workstation
If the Remote Desktop window is not in full-screen mode, Alt+Control+Delete will control your own Windows workstation.
On a Windows workstation press the Alt+Control+End keys to send Alt-Control-Delete to the remote server.