XP and similar OSes could be configured so you had to enter Ctrl+Alt+Del before you could log on. It's called a 'secure attention sequence' [1] and the idea is it makes sure the user only enters their password into the login box - not a fake login box on a full screen website or something like that. Fairly easy for a joker in a school computer lab to do that.

At least one computer has been released with a dedicated secure attention button - the NEC Versa LitePad was a ~2003 tablet running Windows XP. It had no physical keyboard, relying instead on handwriting recognition or an on screen keyboard, but it had a dedicated button with a picture of a key on to begin the login sequence. It's the middle button at the top in [2].

(if you're wondering, the other buttons were rotate screen, escape, Fn, up/down, power on/off and wireless on/off)

[1] https://en.wikipedia.org/wiki/Secure_attention_key [2] http://digichar.com/imgs/a/b/i/w/q/nec_versa_litepad_vl___pa...