Log on/off Screen?

Why is it that 2 weeks ago that my computer automatically set a Logon Screen on its own?...When I Press Ctrl+Alt+Delete also it takes me to a little screen that asks to Logoff, Shutdown, Lock computer, Change Pword?...And when I click Start and then shutdown another screen comes up asking me to select Shutdown, Restart, log off, Standby...Its a different screen than usual, when you click the check mark then a list shows up...the computer did this on its own, How can I remove it?....