HI,
we are using server based profiles and so if I want to install IZArc with the switch /qb it gives me the information that the program is already installed and if I want to reinstall it. Well, so its not really unattended. Is there any other switch to bypass these question?

Well, that is hard to answer since IZArc that I get from the IZArc Website uses Inno Setup installer. Thus meaning that your installer has been repacked to MSI?
For MSI, just type msiexec /? at a commandline to see the switches available.
Example silent installation: