not sure whether its a problem with the drive or a problem with windows media player. Take a can of air and blow some onto the laser of the dvd drive, this will make sure that there is no dirt or dust on the laser.
Once you do that put the dvd back into the drive. Give it about 20-30 seconds and then see if it tries to pull up the movie if it does and your getting errors from windows media player then we know that the drive is working cause its seeing the movie properly.
At that point, I would suggest downloading the latest version of windows media player.
Go here to download the latest version of Windows Media Player 11.
After doing that and installing it, try playing your movie again, in fact try several movies just to make sure. Windows Media player may want to download Codecs, if it does then let it. This will allow it to recognize dvds better and quicker.
If the codecs are updated and you still can't get it to see and play movies then I would suggest using a different dvd player such as Cyberlink PowerDVD located on this page:http://download.cnet.com/windows/dvd-software/