Have you installed the latest updated version of nero 7, as the original version (and several of the later versions) had numerous bugs and problems. All version 7 updates are free if you have a genuine valid key. Latest version is 7.7.5.1 from here:
http://www.nero.com/nero7/eng/nero7-up.php
Because the updates are standalone full versions, you have to uninstall your existing version first.
If you already have the latest update, then it's possible the problem files are corrupted or contain malware, especailly if you got them off limewire or similar. P2P files are rife with viruses and other malware.