If you built this PC, the only way I can see Nero 8 ending up on it is if you installed it separately, as Nero is not part of any Windows Vista disc, at least as far as I'm aware of. There's a good chance that it was bundled with your motherboard's drivers/utilities disc or a disc that came with one of the DVD drives you got for the PC, so it's worth having a look around.
You can download the Nero 8 software from the Nero website on this link.
Before installation, you will likely need your Nero 8 serial #. If you don't have this handy, you can try retrieving this using Magical Jelly Bean (link), assuming Nero leaves the key in the registry when uninstalled.