I don't think you can download totally free DVD software from any legal site.
If you have a DVD player on your computer, I am surprised that it didn't come with some sort of software....Power DVD is a popular one.
You might find some sites which have software that comes with spyware so be careful....!
As a last resort you can try downloading some "free" software from Kazaa etc, but I wouldn't recommend it as there are so many spyware-loaded and virus/trojan programmes on the Kazaa software network.
Good luck.
