I don't know why, but some msi's doesn't support the /qn switch. For example, Adobe Acrobat Pro 6 and PC-Cillin Internet Security 2005. They can only be installed by /qr. Anyways, have you tried using msiexec /i setup.msi /qn ? Or instead of using /qn, just use /quiet. If your msi is version 3.0, /quiet should work.