account lock out

In this W2K Domain using AD, some users lock there account at the end of the day rather than logging off. For some users this is fine but this prediciment is slowly spreading around the office ??. The user would log in with the correct ID credentials, then they would proceed to open MS Outlook, witch then asks for a username password and Domain ? The users password has not been recently changed they can still access network shares as well. the prediciment at present is just logging into Outlook HELP ????????