illegal disc error when burning DVD in NERO7

I have Gigabyte 16x GO-W1608A DVD writer.iam using latest version of NERO. iam having "illegal disc"error message when i try to burn DVD-R. I have no problem with DVD+R.
A few weeks back i had no problems with either +R or -R Disks. i have tried moser baer & sony discs both of which give same "illegal disc" error message. Moreover Nero starts to write at only 4X speed altho the disc is 16X. I have no problem burning the same batch of disc in my brother's computer. I have recently updated the firmware for my DVD writer.
Please help.
Ramkumar