Some User account locked out frequently

Hi all
I have four users in our NT 4.0 Domain who are running windows 2000 pr and xp pro. They can
logon the Domain but after they work on the files
about 15 min to 20 min their account are locked out. The PDC: NT 4.0 SP6A. Windows 2000 sp4. Windows xp sp1. Any clue? Thank for any help!
Gary T Network Administrator TCH