Background network authentication using local user information?

I support an application where the user is generally disconnected from our network during the course of their work. I have a requirement to authenticate the local user to the network when they connect to synchronize their data. I already know how to hit the LDAP server to validate the user when they provide a userid and password. However, the authentication I provide MUST be seemless to the end user (i.e. I cannot ask them to key in an ID and password each time we attempt to authenticate).

Can anyone offer me some insight or example of how to pass to the local account information? I've spent several days reading hundreds of posts and white papers and have not found what I need. Thank you very much for your help.