Well, I think this has little to do with AutoCAD. Also, By "24bit", I think you mean "32bit". My office uses the same approach: keeping a 32bit AutoCAD installed in a box, and users can remote in (use "Remote Desktop") when needed.
One a user tries to log in via "Remote Desktop", shouldn't the user be prompted if other is currently logged in? Then the user can decide if the other current user be kicked off or not (if the other user is an admin, then he/she cannot be kicked out).
There might be freeware/shareware available doing this (or similar). But if you want a quick and simple home-grown programmatic solution, you can easily use .NET to build a windows service that host a WCF service, which supplies current log-in user. This windows service app would be installed in the remote computer. Then you can build a small app that communicate to the WCF service hosted in the remote server to know the remote computer's availability. The app can poll the remote computer periodically and shows the remote computer's status, say, in system tray...