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...