<p>We have a Windows Form application which hosts an IE browser control. Our users run the application and open links to documents stored in MOSS. We are trying to set the application up to pass in credentials of a service account so that we can avoid giving all users access to the MOSS site. We have used code found <a href="http://groups.google.com/group/microsoft.public.windows.inetexplorer.ie5.programming.components.webbrowser_ctl/browse_thread/thread/8a7f5038432f328e" rel="nofollow noreferrer">here</a> which seems to work fine if the user is not currently signed on to our domain. However the application will not seem to pass in the service account authentication for any user which is already authenticated to the domain. In this case it just seems to use the authenticated users credentials.</p>
<p>How can we make this work?</p>

Would impersonation not help here? You can run that bit of code (the form) under the guise of the service account user you're talking about and the browser control should then run within the same context.
the following links might help;
http://msdn.microsoft.com/en-us/library/b80a7e92.aspx
http://www.buro9.com/blog/2006/10/06/impersonating-a-user-in-c/