Win 2000 server

Am using a pc installed with win 2000 as my server(it is not a real server) with clients of win 98 but my clients show me a log in error “there was no server to validate your password” but when i click ok, i can access the sever. After around 10-15 mins when the server has been running i can log on without any error, so what could be the cause of this error??