NT users

Out of the blue, one of my users got locked out from our NT 4.0 server every few hours. I double checked users property and everything looked good. I even put a check mark on Account never expired. Few hours later, this user called to inform thathe got disconnected. I went to check the user property and found a check on Account Locked Out. It occurs 3 to 4 times a day.
I Went to Microsoft website, technet found nothing.

Need all the help for this problem. I ran the antivirus on the user machine and found no viruses.
Thanks,

-Lake