Windows

I've seen this issue before and it has almost always been that the client has a different username and/or password to a share or server.

Does the user have drives mapped to other resources? That would be the very first thing I would check.

There may be a DNS issue with that particular machine's IP address. Try to assign a static IP address to that machine and the problem may go away.

weird. are the pc's time-synchronised to the server. check in the win2003 security log to see if clues in there. do other user accounts have that problem from that box. does problem user account move with user to other boxes.
consider swapping in a known good nic into the win98 box if easy to do? maybe check win98 box for virus and trojans?

Thank you all for your advice. However I am still none the wiser. We are running a single domain, single server, all network resources are shared from the single server. No new network connections are made, only the persistent ones that are connected each day upon logon.
None of the 98 clients register in DNS. The problem does occur with other user accounts on other 98 boxes but this one seems to do it much more often than any of the others. The systems are all 100% clean. They all worked perfectly before on the NT4 domain which was upgraded about 2 months ago, when the problem began ocurring. Sometimes users are unable to log on in the morning if the machine has not been restarted since the previous day, this also results in account lockout. I read something about expiry times on service tickets and win98 not handling the renewal of such, but I can't remember where.
Any further ideas?
Ta once again.