If the software is currently installed on your computer, you can find the key hidden in your system registry. Open the Registry Editor (type regedit in the Start menu).
Searching for public serial numbers for Nero 711100 is highly discouraged for several reasons: I lost my serial number. What can I do? - Nero Knowhow
If you have lost the serial number for your Nero 7 installation, there are several official and safe ways to retrieve it without resorting to dangerous "crack" sites.