CNET Downloads would not, (or should not), include a serial number in their download. It suggests that the download file is incorrect, and yet as it is only an installer, I would not have expected it to have included a serial number.
Try this. Delete all the information from that Customer Details section, eg do not enter a name or Company name, or serial number, etc, then click Next. The trial version and the full version should look the same, except the trial version needs no personal details.
Are you sure it is the Nero 7 Ultra Edition in the link I gave? That file is 170MB and is a large download. If it is not that one, please give us the web site url of the site you downloaded it from.
If all else fails, I would suggest the download is corrupt and you should uninstall Nero 7 and re-download the trial version from Nero's site itself;
http://www.nero.com/nero7/eng/nero7-demo.php
Mark