W2k/Win NT4.5

Converted main server to W2000 4 weeks ago. Other server which is used for Zetafax still running NT.
Suddenly after easter holiday we cannot access the
NT box on a network drive and it says password expired when trying to make connection. Some people are still able to access network drive and run the zetafax client. No restrictions were in place before so no passwords required to run the zetafax client.What is causing password to expire on networked drive and how do we fix? Why does it allowsome people to access?