WSS - Changing the "administrator" username and password

Today we received the call from one of our customers because for some reason their Windows SharePoint Services web site and intranet stopped working. Some days ago they were making some work with Exchange so the first thought was that Exchange could be causing some problem.
After checking from close the server logs I discovered that some errors were pointing some kind of authorization problems. So something changed with the authentication.
IIS was working without showing errors and the web sites looked working... but the browser was showing an error. Then I went checking the Application Pools inside of the same IIS and I discovered that the application pools of each one of the sites were disabled. Inside of their properties in the tab "identity" I found that the user name and password were not updated. The solution was to select a network service like authorized user. Then like art of magic... the web sites apeared online again!