Finally someone with the same problems as I am experiencing!
I have two cases of Nero problems:
1) A PC with a MSI8332A cd-writer. Nero 6 produces only coasters. Nero claims the burn process completed succesfully, but upon verification the files are different. Using other verification tools confirms the erroneous writing. Nero support claims it has something to do with my computer ("since no others ever reported this type of error"). Installing Nero 5 instead solves the problem. No more write errors! I have tried about 5-6 different cd-brands, conclusion: Nero 6 writes only coasters on this computer, Nero 5 works great.
2) A PC with a NEC2500A dvd-cd-writer. Nero 6 produces coasters if I try to write more than about 3.9 GB on a dvd. Same story as above. The burn seems to complete ok, but the verification indicates unreadable files. In fact, all files beyond the 3.9 GB limit are unreadable (tested with NERO CD-DVD Speed)! Tried different dvd brands, including Verbatim media specifically recommended by NEC. No good. Burning less than 3.9 GB and burning cds is no problem. Burning DVD+RW (2.4x) above 3.9 GB limit seems to work ok. Burning DVD+R or DVD-R above same limit (at any speed) does not work.
Operating system on both PCs are Windows XP with all latest updates.