Before you loaded Nero 7 did you run the clean tools for Nero 6?
I might suggest you run the Nero 6 clean tools, and then the Nero 7 clean tools and maybe even the registry and driver remover clean tools and then download 188.8.131.52a and then reinstall. Hopefully you don't have some strange serial number in there.
If you want, prior to all that, see if you have a NeroHistory file you could post. Try to edit it to remove any serial numbers and, if it is large, just include the last couple of burn attempts.
Also, just in case, .iso files are burned using the Recorder-burn image option in Nero Burning ROM. Slower speeds are recommended, like 4x.