Options->General->Lock
From the help file:
Ask for Password
If this is turned on and you have set a password by using the Lock button, mIRC will ask you for a password each time it is run.
The Lock Button
This allows you to set a password which locks the options in the lock dialog.
Note: If you hold down the Control key when you minimize mIRC, it will ask you for the password when you try to open the mIRC window later. If you use Control+Minimize with no password set, mIRC will ask you for a temporary password.