I was having this same problem, and I was logged in as the administrator. I isolated it though, check this out:
It turns out that when I had audiograbber running, Nero would act like I don't have burning rights. It's very easy to reproduce: open audiograbber and start Nero, and Nero tells me I don't have burn rights. Close audiograbber and I'm fine. And even if I already have Nero open and burnable, if I open audiograbber I can't burn.
I'm on Win2k SP2 using Nero 18.104.22.168.
So maybe if you're having this problem, shut down absolutely all other running programs?
I won't be checking this forum again, if you have any questions email me firstname.lastname@example.org
. I'm going to CC nero tech support.