I found this handy little tip today when I had to send CTRL+ALT+DELETE to a remote desktop session (which always ends up being processed by the local machine and not the remote one). The keystroke is CTRL+ALT+END. Handy to know and remember!
Shift+Ctrl+Alt+Delete for VNC.
Ctrl+Alt+Del for Local system.
Ctrl+Alt+End for Remote Desktop.
Ctrl+Alt+Insert for VMWare image.