You probably don't need to log in as a user, which is fortunate, since it's bad design and probably not possible without a lot of hacking.
Take a look at how your Linux machine starts up gdm/xdm (probably via init and it's config file, inittab). It should give you some ideas about how to implement this.
Basically, your system will start an X server and then run rdesktop on top of it. No login will be needed, since, effectively, root will be telling it what to do, but doing it as a different user.
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Bitt Faulk