my mac mini will not play dvd's

I've owned (and loved) my Mac Mini for several years. When I first bought it, I tried to play dvd's, but they'd spin for a while and then pop back out. I bit the bullet and bought a dvd player anyway. I'm now in the process of getting rid of a bunch of stuff I don't need, and I'm re-investigating the possibility of using my Mac Mini to play dvd's. Do I need a new drive or new software?

Thanks!