I downloaded the trial from the adobe web site, and unpacked it. Opened it, and clicked the setup file. It starts the installation, and immediately tells me to "Instert Adobe After Effects into Drive C:/"
You have a messed-up install somewhere, possibly from another Adobe tool. Run the CS3 Clean scripts before attempting another one. Also make sure that whatever other tool may be causing this error has indeed finished its install, which can only be guaranteed by logging on as administrator. Likewise, you should install all Adobe software with full administrative privileges. You can also manually verify active instances of setup.exe and msiexec.exe in your task manager. Simply kill them before launching the AE install.