Typically with remote desktop clients there are custom keyboard shortcuts for sending the ctrl+alt+del keyboard combination. They do this because this combo typically is captured and used by the client Operating System. I made this assumption when using rdesktop on linux, and it took me a little while to search and to find out that this isn't the case. The client rdesktop under linux passes ctrl+alt+del directly through to the server, so there is no alternative shortcut needed.