OK, reading this as somebody with only basic knowledge of the Windows forwarder install, it looks like you have /qn as parameters to msiexec, but I think the correct parameter to the splunkforwarder itself is /quiet according to the manuals. Could it simply be the installer wants to show you a user interface but msiexec is blocking it from doing so? Is there nothing in the logfile you specified (C:splunkinst.log)?
I'm guessing you already know, but for completeness sake the manual for deploying the forwarder from the command-line can be found at http://docs.splunk.com/Documentation/Splunk/latest/Forwarding/DeployaWindowsdfviathecommandline.
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