No idea why Nero is crashing. You might try uninstalling Nero entirely, then run the Nero Cleaner tool for Nero 7, then reinstall. No guarantees for that helping though.
Or you could try a different program to make your dvds. If your mpeg files are already dvd compliant in all specifications (resolution, frame rate, type of audio), you could use a dvd authoring tool like GUIforDVDAuthor or DVDStyler. If they have to be adjusted—reencoded—you could try DVDFlick or FAVC. All of these programs are free to download and use.
What program did you use to convert your AVCHD files to mpeg2? Ulead Studio 11 should be able to take AVCHD files and output to dvd for you if you want to do this in one step. It isn’t a free program, but there is a trial available to test if it will work for you.