If you change the password or user name that you use to log in to your computer the CasaTunes Service will not start up. The easiest way to fix this is to reinstall CasaTunes from CasaTunes.com/downloads and during the installation change the user name and password where it asks you to verify your logon information.
For the computer nerds out there, go to services.msc, double-click on CasaTunes Windows Service and change the information in the logon tab to match the new username and password. Then simply restart the service.