I’ve seen on these forums that a lot of people have had an issue with the built in DVD/CDRW drive where CDs work fine but DVDs don’t play.  I ahd this same problem, although mine was probably caused by my brother dropping the machine.  (I didn’t mention that when talking to customer service).
  I called Sony and they tried all the software-based methods of fixing the issue, (re-installing the driver, system restore, etc) but none of those worked, so they told me that they would do the repair.  They sent a box for my machine and I sent it in, and within a week I had my computer back, just fine, with the drive replaced.  I would suggest that anyone who has this problem and who has a machine still under warrantee call customer service and arrange for the repair.  It was certainly worth it for me not to have to buy a 400 dollar replacement and do it myself.