Start and stop processes do not work.
Only the user who installed the application is able to start and stop the service. Probably you are launching the GUI using a different user. Try again changing the user or reinstall the application with the corresponding user.
You can try to start and stop the Lan Core service manually. In Windows XP, go to Start menu, My PC, press the right button of your mouse and select the option Manage in the pop-up, go to Services and Applications, Services. There you will find a service called Lan Core that you can start and stop manually.
The thin client boots but the system never finish its initialization (a black screen remains).
That's probably because the video card was not properly detected and the X Window System cannot start. Change the device configured in the GUI for this client (different devices include different video card and network drivers).
The thin client boots but the system shows a blue screen with a message within a white box.
DHCP and TFTP services work properly since the thin client boots. Therefore, the problem is due to: (1) the server host cannot be reached from the client, or (2) the remote desktop protocol does not work properly in the server. If (1) check the server IP address and take care that it is correctly configured in the client configuration using the Lan Core application. If (2) probably have not configured correctly the RDP server in your Windows, try to connect manually using other computer using a remote desktop connection.