This is easy but not obvious.
To Play ISO File from Hard Drive:
Step 1. Rip the DVD to an ISO file, using a ripping/reading tool. If the original disc has copy protection, you'll require use of a tool like DVD Decrypter or AnyDVD to rip the disc.
Step 2. Use a virtual CD/DVD mounting program, like the freeware MagicDisc, Gizmo Drive, or Daemon Tools Lite. Mount the ISO file as a virtual CD/DVD drive.
Step 3. Play the video with your favorite DVD player software. For example, the freeware VLC Media Player.
Done.