What you might want to do is use system policies to prevent the user from entering the network properties via Control Panel.
However, you might be able to use a registry hack to force a computer name. I'm now using Windows 2K, but the registry key should be similar for Win98. In Win2K, it is:
HKLM/SYSTEM/CurrentControlSet/Control/ComputerName
In this key, you will see two sub keys, ComputerName, and ActiveComputerName. These are string values, and they should be the same. In my Win2K registry, I also see HKLM/SYSTEM/ControlSet001, and HKLM/SYSTEM/ControlSet002 - however, I believe the Win9x OS only has a CurrentControlSet, so I would start looking there.
As registry editing can be dangerous - BACK UP THE REGISTRY FIRST! Also, I would recommend enabling a policy to keep the user from changing the computer name, and use a registry hack as a last resort.
Good luck!