You may find when using some editions of Windows Vista that your password expires and you are required to change it. This problem will keep re-occurring until you change your account profile to prevent password expiration.
To do this, start from the run box (Press WINDOWS KEY + R to access this):
Once the Local User Manager has loaded, click Users in the left column and then double click your username in the main window:
Now, tick the Password Never Expires box followed by OK:
In future, you password will not expire. Allowing you to log on without needed to change your password again.