While most PCs that come with DVD drives include some form of DVD playing software, if you added the DVD drive, you may find your PC unable to play DVDs. This can be remedied with free downloadable and legal software. This article describes one method that works on a Windows PC, however, alternate methods that work on both Windows PCs and other platforms including Macs are available in the Related wikiHows section, below.

[edit] Steps

Make sure you have a DVD-Rom or DVD-RW drive. A CD-Rom or CD-RW drive will not play DVD media. Look for the DVDRom symbol on the front of the drive
Install a free media player program (e.g., VLC). You can also use Windows Media Player, which comes installed on most PCs and is simple for beginner users, but requires extra software to play DVDs and many other video formats.
If you are using Windows Media Player, upgrade to the latest version. Go to the help menu to find a link to the latest upgrade, currently version 11.
Install an MPEG 2 codec. This is the file you need for Windows Media Player to know how to interpret the files on the DVD.
Download a free MPEG 2 codec. While there are several versions, including Stinkycodec that are purportedly functional and free, many people find they don't work.

[edit] Tips

Another alternate method to enable DVD playback is to download a package like "K-Lite Codec Pack" which not only adds support for DVDs but also adds support for playing just about any format of media, or A/V file out there.
With VLC, you do not need to download any codecs or anything else. VLC can play basically any audio or video file.