First of all, you want to be sure you know whether it's alocal password
that's expiring, or a server one. It all depends whether the computers are
members of an NT domain, or not.
If it's the network password, then you need to open "User Manager for
Domains" under "Adminstrative Tools" on the NT Start Menu. Changing the
password for a user is fairly self-explanatory.
On the user-manager menu you'll also find the Account Policy settings, where
you can banish the password-expiry for good.
It's a good idea to have a wrong-passowrd lockout set (say after five tries)
so that brute-force attempts to hack the password will fail, but I question
the usefulness of password-exiry unless it's a very short interval (in which
case it's a PITA) - If a hacker has 42 days worth of access I'm sure he will
have got bored by then anyway ;-)