I am having this exact same issue on my Lenovo E520. In the Advanced User Accounts - the box to check to "require users to press ctrl-alt-del" is greyed out. In the Local Security Policy I have disabled the Interactive Login. With these changes made the computer will require the Ctrl-Alt-Del during the first reboot but will revert to the factory settings upon subsequential reboots. I believe it is caused by the fingerprint scanner software or drivers
but the uninstall option is not available in the Delete Programs utility. Also I am leary on removing these as I don't want to cause conflicts with the other Lenovo factory installed software. Any suggestions?