When MUST_CHANGE keyword is used user is forced to change the password when he/she tries to login for the first time.
CHECK_EXPIRATION and CHECK_POLICY are only enforced on Windows Server 2003 and later.
If you want to turn off the password expiration enforcement or security policy enforcement, you can do by using the following statements. (But these wont work if you have created your login with MUST_CHANGE and user didn't change the default password)
ALTER LOGIN sql_login WITH CHECK_EXPIRATION = OFF
go
ALTER LOGIN sql_login WITH CHECK_POLICY = OFF