Don't try to install Nero7 Demo or full version on top of Nero 6.
First uninstall the previous version which is already on your PC and then try to install the Demo version.
IF your previous version of Nero 6 was not a OEM version but Retail version then you can use the same key to install Nero7 Demo. On doing so, Nero 7 Demo will become full legal version.
(NOTE: only Retail version key works with Nero 7 Demo)
There is one more method to check your key for if at all it will work with Nero 7 or not. Just go to
www.Nero.com (from where you downloaded the Demo file) and enter your Nero 6 key there, you will automatically qualify for a FREE upgrade.