You can change a registry value HKEY_LOCAL_MACHINE\ Software\ Microsoft\ Windows NT\ CurrentVersion\ Winlogon\ PasswordExpiryWarning to change the default period in Windows 2000, but it’s better to apply the change through a group policy to allow the change to be applied and adjusted dynamically. You can apply the policy at the site, domain, or OU levels through the appropriate Active Directory console. To change the policy locally, choose Start, Programs, Administrative Tools, Local Security Policy. Expand the Security Settings/Local Policies/Security Options branch and set the value of Prompt User to Change Password Before Expiration as desired.
I highly recommend you do not disable password expiration though. Helping with password changes can be a bit of a hassle, but nothing like the hassle of having your network insecure...or breached.
Take a look here for further password related assistance:
http://www.labmice.net/Windows2000/Administration/password.htm
Best resource I know for password configuration and trouble shooting information.
Hope this helps.