You don't need a serial number in order to install and run Sony Media Manager for PSP, because the utility is now free of charge. I suggest you to remove the software, delete the installation package and navigate to the official web page. Click the Download button below Media Manager for ACID Pro 5 and Vegas 6, save the file to your computer and install it following the provided instructions. After that, go to Computer > (C:) > Program files > Sony Setup > Media Manager for PSP, double-click the setup.exe file and install the software. If you have issues, you can get in touch with the Support Team using the contact information from their website.
