You have to do a thorough job of uninstalling ECDC first. After running UNINST, you then should reboot, and manually make sure ALL ECDC files are gone. Then run WinDoctor (NU) to make sure all relevant registry entries are deleted. They never are. When all is clean and green, then install Nero 5.X. Should be no problem. But if any vestige of ECDC remains, you could have problems because they are not at all compatible.
Restarting in Safe Mode usually means you had a start that didn't finish, and then a restart.
I would do a shutdown and then reboot. It is sometimes cleaner than a warm restart.