How do I force users to press CTRL + ALT + DELETE in order to log in?

You can force users to press the CTRL + ALT + DELETE key combination to log in by modifying group policy.

Press start
Type: local security policy
Press enter
Expand local policies
Click Security Options
Double-click "Interactive Login: Do not require CTRL+ALT+DEL"
Check Disabled
Click OK

This will require users to press CTRL + ALT + DELETE before being presented the welcome screen. The welcome screen will still be displayed and will allow the user to select their account from a list.

If you would like to remove the list of accounts and force the user to type his or her username and password, you will need to change another setting inside the local security policy window:

Double-click "Interactive Login: Do not display last user name"
Check Enabled
Click OK

With both of these settings in place, the same functionality of the old-style "Press CTRL + ALT + DEL to login" window is achieved.