1: Optical drives, and especially burners, fail pretty often
2: For a while Dell would ship systems with DVD+R only drives. So, if you have DVD-R (note the - instead of +) media, it won't work in a +R only drive.
First thing I'd do is check whether you have a +R only drive, and if it's a +/-R drive, then it's probably failing and needs to be replaced.
I have a Dell Dimension 4700 w/XP. I bought some Memorex DVD-RW discs to make DVDs, but when I put a blank DVD in the drive the system/computer doesn't even recognize that it's there.