Symptom
When you try to install QuickTime VR Authoring Studio, this message appears:
"Installation on the active startup disk 'Macintosh HD' cannot take place while another application is running. Please quit 'compressord' and try again."
Solution
Restart the computer, then install QuickTime VR Authoring Studio.
The process "compressord" runs in the background and starts when you use the Compressor application included with Final Cut Pro 4 and DVD Studio Pro 2. Restarting the computer quits the process and allows you to install QuickTime VR Authoring Studio.