Ctrl+Alt+Del invokes WinLogon, the highest priority process, that then delivers the task manager. I believe this is a kernel invocation, so regardless of user land dysfunction, it will always work. Tthis is possibly attributable as much to convenience as it is to security -- the Secure Attention Sequence[1] is intended as a login-spoofind protection which will suspend other tasks (like a spoofed login screen) before continuing forward with user auth.