I have a Samsung R480 running Windows 7, with a DVD-RW drive. It can read CDs fine, but won't read DVDs. It used to be able to read both, but it doesn't do so any more. It would register the DVD every so often, but most of the time, it does the loading thing for a minute or so, then fails. If it helps, the driver lists it as a CD-ROM drive.
In case anyone runs into the same problem, I resolved it by bringing it back to Samsung, where they replaced the drive. It took them a bit over a week, mostly cause they were waiting for parts, but partially cause they were having trouble locating the hiccup glitch in my system. It was free cause it was still under warranty, but it sucked that I had to wait that long. Also, I would have liked to credit this answer to warrenkopp, but he posted his response as a comment. Still, thanks for all your responses, especially warrenkopp.