If the drive is able to work with CD but not with DVD it is possible that the DVD laser is damaged. In fact, there are two separate lasers in the optical pickup, so if one becomes damaged the other can still work perfectly.
Just to exclude some sort of software issue, can you test the drive in a different computer? If in the other computer the drive works normally, then there is something wrong in your machine.
If the drive is able to read DVD but not to write them, then it is possible that you are using crappy discs (are you using Princo? :eek:). Try to use a proven quality disc like a Verbatim. If this solve, then it was a media issue, and the solution is to not buy anymore low quality media.