I think you should have a look at www.installshield.com to get knowledge of how to create a silent install with installshield.
Which makes it easier and you don't need to use that setfocus thing.

Unextract and use setup -r to create an setup.iss file in %WINDIR% folder. Copy that to the extracted folder and next time run
setup /s instead of setup. Then it uses the recorded setup.iss file for silent install.

Fabian