How do you watch a movie DVD on your computer?

Insert it in DVD driver and it will play with the media player you set for DVD player. Like powerDVD, kmplayer.

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Some Intel/Windows machines came from the factory with a DVD decoder included (Windows98, ME, 2000 and XP). If the DVD doesn't start on it's own, check to see if the decoder was included in any discs that came with the machine. If not, there are freeware and retail programs available for playing the disc.

Any recent Mac (since OS/X) will fire up and play 'DVD Player' automatically when a disc is put into the drive. Before OS/X only a few Macs could play/use DVDs.

Some Unixes and Linuxes included DVD support, others do not. Check the computer's documentation for more info or the manufacturer's website.

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In order to play a DVD, your computer needs to have a specific set of instructions in order to decode the compression used to store the images. These are typically referred to as codecs (Code/Decode). There are some free codec packs that will allow you to play DVD's on your computer, but newer drives incorporate copyright technology that may cause that to fail. The simplest answer is that you need DVD playback software, such as WinDVD or PowerDVD.