Every now and again you'll need to send Control + Alt + Delete to a RDP session. You can't use the actual keys, as Ctrl+Alt+Del is a special shortcut key combo that will end up running locally. Instead, use the following key combination in the RDP window:

Ctrl+Alt+End

The RDP client will translate the Ctrl+Alt+End shortcut to Ctrl+Alt+Del on the remote machine.

For more shortcuts, see the RDP Shortcut Keys topic.