That is a simple question with an interesting answer. There's no plain black screen with a login box like under XP you can have it default to, but you can opt to force users to type in a username and password instead of having a list supplied. Odd way of going about it though. If you go into the Group Policy Editor under Windows Settings->Security Settings->Local Policies->Security Options look for the Interactive Logon policy "Do not display last user name." If you enable it not only will it not display the last username, it won't display any usernames, forcing the user to enter one manually. Not the most logical title, and not in the description, but that's how it can be done. The policy right below it lets you force the user to press ctrl + alt + delete to logon, which you may also be interested in.

Hope this helps,
John