Any device could cause that sort of behavior, really. If you suspect Nero, and you uninstalled it, perhaps it changed a setting on the DVD drive that hasn't changed back.
Here's a few ideas:
1. Vista is unable to turn off some device. Some hardware device is running and Vista has lost the ability to tell it to stop. (is there a disk in the DVD drive? USB device?)
2. Some software is running that Vista can't control, such as anti-virus.
3. A sleep mode setting was changed. These may be found in your BIOS under "power" or some such. (You did not list your motherboard.)
4. Check in your control panel for an item called "Problem reports and solutions" if there is a report generated each time you shut down, what does it say? If not, that's a clue as well.