/Q /NORESTART

For progress bar only: /qb /norestart

Microsoft Security Essentials

setup.exe /s /runwgacheck
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To eliminate the update check after install add this command:

taskkill.exe /f /im /msseces.exe

so the install script should look like this:

mssefullinstall-x86fre-en-us-vista-win7.exe /s /runwgacheck
taskkill /f /im msseces.exe

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This works for me:

mssefullinstall-x86fre-en-us-xp.exe /s /runwgacheck /o

This will only do the installation. It won't update or start mse.

If you want to start and update then run this after the installation:

msseces.exe /update -hide