Accounts get locked out for no reason

I am currently running a w2k server environment with a mixed client base of w2k pro and 9x boxes. About 4 of my 9x boxes get locked out (for no reason according to the users) about 2 times a day. I have tried installing the directory services client on the systems but it did not fix the problem. The lockouts seem to hapen when the user is not using the system for an extended period of time. When they come back they start getting access denied messages and have to log off, call me, I reset there account and then they are good for a few more hours.