Is there some service or application running on the Win98 client which would cause this to happen? The logon attempt always fails, and it appears the SYSTEM account may be involved. It is always listed as EVENT ID 529 and the cause for the log on failure is bad username or password.
Is there a way to get the Win98 machine to stop trying to access the NT servers?
From a command prompt, type NET USE and see if there are any entries. You may have a connection to IPC$, which is the secure channel used to pass credentials.
You might have left the workstation as a domain member. In the properties of the Client for Microsoft Networks, check to see if Log Onto Windows NT Domain is checked.
If all that comes up negative, you might need to run Network Monitor on one of the domain controllers and sniff for the traffic that comes from the client. The port number and type of request might give you a clue as to the service that's doing the request.