You can install the Scalyr Agent silently (via its .msi package) in Windows with the following command
msiexec /a "ScalyrAgentInstaller-X.Y.Z.msi" /QN /L*V "C:\scalyr_install.log"
The options are as follows:
/a :: run Administrator install sequence
/QN :: silent (no UI)
/L*V "C:\scalyr_install.log" :: verbosely log to the specified file