In order to send Ctrl + Alt + Del to the remote PC, just press the following alternate key combination:

Ctrl + Alt + End

there are a bunch of other keyboard shortcuts you can use in Remote Desktop:

Alt + Page Up – Switch between programs (Alt + Tab is the local command)

Ctrl + Alt + End – Display the Task Manager (Ctrl + Shift + Esc is the local command)

Alt + Home – Brings up the Start menu on the remote computer

Ctrl + Alt + (+) Plus/ (-) Minus – Minus takes a snapshot of the active window and plus takes a snapshot of the entire remote desktop window.