It is very possible that in your travels of software on your computer, you have installed something on your computer that has installed an Upper or Lower Filter driver that is impeding your ability to read and DVD's.
Have a look here
and use either the "fix it" or the manual instructions for removing the Upper and lower filters in the registry.*CAUTION*
The manual fix requires manually editing the Windows Registry on your computer. Done improperly, registry editing can render your computer a brick requiring you to either roll back using system restore or even reinstall your operating system.*end public service message*
Hope this helps,