I don't know which OS you're on, but I think it's pretty safe for me to assume that if you're having problems watching a DVD on your computer, the pre-installed media players on your computer are lacking the proper libraries (i.e. if you want to use Windows Media Player or RealPlayer, you'd need WinDVD or some similar proprietary libraries to allow you to use those programs for playback).

Never fear--there is a wonderful free, cross-platform solution, and it is called VideoLAN Client. Find the right version for your OS here: http://www.videolan.org/vlc/