Try to get your serial number now, before you do anything else. Look for a NeroHistory log to see if you have one anywhere. Maybe you saved an error log somewhere. If you don't have one of those, check out the link below in case it is still in your registry.
If you can't find it there, in Vista it is at the location below and might be in WinXP
If you can find it or have it somewhere, you can download an update.
Search your hard drive for anything that says Nero. Maybe you will find the download, but you will still need the serial number.
Also, it looks like SetupX is the ProductSetup. If you can get that to run, look under License. Did you update from the Product Setup to start with or what?