Buffer underrun error occurs when there is insufficient CD Writer memory (not system memory)available to support the volume of the files/music to be burned, especially when there are large files to be burned.
YOu need to increase the data buffer size (for Nero, it is set to automatic but you can change it to manual), maximum buffer allocation for Nero is 80MB, sometimes it works, sometimes not especially if you have a lot of applications running in the background.
1. Close all other programs running in the background.
2. Dis-able anti-virus program, make sure though that your file is not corupted or has no virus in it.
You can also try to lower the burning speed, avoid using additional features like hiss reduction, equalizer, normalization, karaoke function, etc. during burning, it will help.
I tried it before when I have my former CD burner. Don't try to use RAM clearing softwares, it doen't help a lot. Make sure to make a CD image first, else, you will lose a lot of CDs, actually, sometimes the error occurs during the burning process even though it did not report a buffer under run error while a CD image is being created.
Newer burners with a speed of 48x and up like Samsung and Lite-On, have buffer under run protection feature (the burner automatically does a memory reset during burning). If you have the budget, try to replace your burner, I have a 52x32x52 burner with buffer under run protection now, and oh boy, I had forgotten the buffer under run error dis-appointment, had saved a lot of CDs too.