>How do you prompt the nt authentication login screen from with in your app while the app is running. can it be done and if so how? Will it change the sys(0) information on my machine and how?
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The problem that you face is that if it does anything but permit impersonation, it'll log out the current user - essentially, there's no way to know that any files open under the current userid are valid once the new userid logs in, so it should terminate your app, or at least break all file connections, since under NT, a local user might well not have rights to all local resources.

There are procedures to perform user validation therough NT; I'd strongly recommend investigating the availability of third party packages or ActiveX controls for this.