Have you run any lens-cleaning software in the drive lately? You know, the CDs you put into the drive that has brushes on it, that wipe the laser lens...
Have you tried updating the driver for the drive lately? Click Start, type Device Manager, when it appears in the menu, click on it.. When Dev. Manager opens, click on the "+" sign next to CD/DVD rom drives.. Right click on the problematic drive in the submenu, and click "update driver software"........If there is an updated driver available, let windows install it and try some DVDs again..