Diacos

First, run the installer with /r flag - answer all questions. It will save the answer file to C:\Windows\setup.iss.

Copy that file to where your Diacos installation is located.

Silent installer for Diacos will look like that:

<?xml version="1.0" encoding="UTF-8"?>

<packages>
    <package
            id="diacos"
            name="diacos"
            revision="1"
	    reboot="false"
            priority="1">  

	<check type="logical" condition="or">
		 <check type="uninstall" condition="exists" path="ID MACS Client 6.72.000 Client" />
		 <check type="uninstall" condition="exists" path="ID DIACOS® Client" />
	</check>

<install cmd='%SOFTWARE%\diacos\idmacs\clientsetup\setup.exe /s /sms /f1%SOFTWARE%\diacos\setup.iss /f2C:\Windows\diacos.log' />
<upgrade cmd='%SOFTWARE%\diacos\idmacs\clientsetup\setup.exe /s /sms /f1%SOFTWARE%\diacos\setup.iss /f2C:\Windows\diacos.log' />

    </package>
</packages>

NOTES
Diacos uses InstallShield installer. It won't work silently for SYSTEM user unless you add a /f2 flag (used for a log file). Interestingly, the installer works silently as Administrator without this flag...