login

I am facing one problem, when I install and configure windows 2000 professional and I get that machine to become a member of windows nt server, then from server i create a user to login from that windows 2000 professional, it gets loging successfully into it and even he saved his files on the server also, but after few days from client side he is unable to login into that machine as a normal user, but as a domain admin or local admin he can login easily.
So what should be the problem exactly.