On the Windows 98 CD-ROM there is a folder with the WIndows 98 resource kit, Which includes policy editor for windows 98. Install the polciy editor on you computer. Create a policy save it on your computer.
In the policy make sure that you enable the option: must be validated on domain to access windows.
Then make sure that on all win9x computers you enable the check box logon to windows nt domain contrler in the network properties.
Then make sure you save it to the netlogon share on ALL Domain Controllers, the default path for the netlogon share is Winnt\system32\repl\imports\scripts it has to be saved with the name of CONFIG.POL.
If you need further instructions on how to create a policy for Windows 9x machiens lookup help in the windows 98 resource kit. It has complete step by step instructions on how to do it.