Friend of mine brought me a PC with exactly the same problem. He thought the Re-writer had failed because Nero 7 Premium kept creating "coasters". Replaced that but it made no difference. Doubled up the RAM with a spare stick of 512Mb and managed to burn one out of three discs at half writing speed. Simply couldn`t do anything else with the computer whilst Nero 7 was running because it crashed.
I then installed Nero 7 on my machine with the same result. We both reverted to Nero 6 pretty fast!