To download and install CS5.5 Pro, make sure you're downloading it from a reliable source, as Adobe has discontinued support for older versions. If you're using an official installer and getting a "Not Installed" error, try running the installer as an administrator, disabling your antivirus temporarily, and ensuring you have a compatible operating system. If issues persist, check if any Adobe Creative Suite remnants from previous installations are interfering, and uninstall them before trying again.