Did you just buy new media for it or are you using a blank from the same batch you made your previous successful burn?
Is it possible your drive only supports DVD-R and the disks are DVD+R, or perhaps they're dual-layer media and you've got a drive only capable of burning to single-layer media.
Did you upgrade to Win7 or was it supplied with the machine? That would suggest that the machine is recent enough for my suggestions to be very unlikely.