[TheMamba]

Could be:

-Bad Media (discs)
-Bad Burner that needs to be replaced
-Your hard drive may be too full. How much free space is on the drive?
-Your hard drive may need to be defragged
-How many other programs do you have open at the same time? Try to have only Nero open

Free up as much space as possible on the hard drive, then defrag it and try again. If you have a different type of media try those.

If you can give us more details, we'll be able to help better, like inside'eye said.