ctrl+alt+delete issue

[Sawer]

since update the last big upgrade my C+A+D doesn't work. When I use the keyboard to shutdown/reboot the system monitor window opens. I was able to get the key combination to work once by going in the keyboard short cuts, 1st had to assign a different combination then back to Ctrl+Alt+Delete, but the next time I rebooted it was back to the system monitor window. Is this a bug in Mint 10- or just in LMDE or is it my system only and how might I fix this?
Thanks you

My bad I had Backspace but it's ctrl+alt+delete that is the issue