I found this in a few different places around the 'net - hopefully it's helpful:
Stop Password Expiration
After you have run Windows XP for a while, you may receive this message when you log on: "Your password will expire in 14 days.....". By default, Windows XP is set up with passwords which will expire after 42 days. And 14 days in advance, Windows will start warning you of this fact.
- Go to Start > Run > type control userpasswords2
- Select the Advanced tab in the User Accounts window.
- Press the Advanced button below the Advanced user management header.
- Select Users in the Local Users and Groups.
- In the right pane, right-click the user name for which you want to change the setting, and select Properties.
- On the General tab, check Password never expires.
- Click Apply and OK (all the way out).