First thing Iwould do is try some other DVD discs. Try a diffferent brand.
See if you are still getting the problem. Some writers can be fussy with their media.
Next if you still arent getting anywhere DVD writer drives are cheap these days.
http://www.ebuyer.com/product/138750
Fitting is easy. Just make sure you ground yourself by touching a radiator or leave the Pc plugged in but power off, and touch the case to ground yourself.
The old drive should slide out or might be held with screws. Unhook old drives data cable and power cable. Slip in new drive, hook it up, and Windows will recognise everything in the majority of cases and you will be done.