You have other Adobe software running background processes (e.g. Adobe Bridge is a common culprit). Identify these in the Processes tab of the Task Manager and end them. If you have other Adobe applications running, close them.
You are installing from the incorrect DVD - either trying to install the Mac version on a PC or vice-versa. Or you are trying to install from the Additional Content DVD before installing the Application.
Cheers, -- Neale Insanity is hereditary, you get it from your children