Before installing SharePoint it is important to give some thought to the account that will be used as the portal administrator. Not only is the admin account referenced throughout the portal (area/site ownership, document metadata, list properties, etc.) but it's also used for many basic functions that are central to portal operation - connections to the configuration database, content crawling, application pool identity, and so on. Because the portal administrator must have complete and total control over the SharePoint server(s), a good practice would be to use an account specifically for this purpose that has limited rights elsewhere in the domain.
Assuming that you have followed this methodology in your portal deployment, the next obvious question is: What happens when you change the portal administrator password? Alas, this is a simple question with a not-so-simple answer. There are several steps required to successfully implement this change and they must be done in a specific order. Download the following article which outlines the scope and implications of modifying administrative account properties and defines a procedure for implementing changes.
NOTE: This procedure has been tested for accuracy but each deployment is different; if you run across any variations please post comments and I will update the document accordingly.
Modifying Administrative Account Properties (PDF)