A Windows 2003 domain controller defaults to a 15 day password reminder unless otherwise configured.
A Group policy can be employed to change this to a more reasonable setting. Most users prefer it to 5 days.
On the domain controller, click to Administrative Tools, and open Active Directory Users and Computers.
Right Click on the domain name and go to properties
Move to the Group Policy tab, and select the policy you want to modify and click edit.
Navigate to :
Computer Configuration -> Windows Settings -> Security Settings -> Local Accounts -> Security Options
Scroll down in the right side pane to the entry:
Interactive Logon: Prompt User to change password before expiration
Double click on it.
Check the option that says 'Define this policy setting'
Set it to the number of days you want the reminders to start.
Click OK, and the policy is changed.
It will take 15 to 30 minutes for it to be propagated out to all of machines on the domain. The impatient can open a command prompt and type: GPUDATE /FORCE to make it happen immediately.