As far as I am aware you can't do what you want easily without a replacement GINA (up to Vista) or a logon provider (since Vista), unless you enter the credentials on the SAS (secure attention sequence) desktop.
However, if you are using a Windows version up until Windows 2003, you could use a canned version like AgreementGINA2 (use a web search to find it). On Windows versions starting with Vista the GINA has been replaced by logon providers, which is a different kind of "program" than the above.
For all I know, some flavors of VNC and similar tools will, however, allow you to send Ctrl+Alt+Del in order to get to the SAS, if that is sufficient for you. But your question sounded like you actually want to automate it.