In my experience, buffer underruns occur when the cd-writer cannot keep up with the cd-reader. If you've been successful before, then it might be that your PC is behaving differently - like there is a daemon running that consumes cycles and resources preventing Nero from working normally. If that's not the cause, try running the CD-writer at a slower speed.

I assume that we're not looking at a Nero spoiler when the free trial period runs out? (I don't know how Nero protects itself after the free trial period).