As long as the OS itself isn't replaced or compromised, CTRL+ ALT+ DEL will protect you from false login screens. The real login screen doesn't do that it just stays there on the screen. But, if you're in the habit of always pressing CTRL+ ALT+ DEL before logging in (there is an option to always require it for the legitimate screen), pressing the SAS on a false login screen will bring up task manager or the secure attention screen (with the log off, change password, etc options). There's no way to tell that it's a fake just by looking at it. One security issue it addresses is that of a false login screen: consider a screen that looks exactly like the normal Windows login screen. The whole point of the SAS is that it can't be intercepted or stopped by user programs.
It is the one key combination that is guaranteed to get the OS's attention.
CTRL+ ALT+ DEL is the secure attention sequence of Windows NT (and its derivatives like Win7).
