Due to your company’s password policies, you might have to change the password of the domain account used as the startup account by SQL Server services.
Your changes will be reflected in the active directory. Next time, when SQL Server services try to start, they will fail to authenticate and will not start.
That is why it is imperative that once you change the startup account password in AD, you also change the password of the account in Configuration Manager.
Refer to this excellent article in BOL that talks about how to change it and what precautions to follow when you have to change the password in AD.
A few days ago, I blogged about how to change the startup account of SQL Server 2005 services using Configuration Manager. You might want to refer to it as well.