How to play DVD, MPEG2 video files?

To play a DVD you need a software DVD Player, then it is just to insert the DVD in your DVD-ROM and hit Play using any player like  WinDVD.
To play MPEG2 with Windows Media Player you need a software DVD Player installed or a MPEG2 Codec.
To play a copied/ripped DVD from your HD you can use  Media Player, choose to open a directory and select the VIDEO_TS folder. Or use WinDVD, just right click on the play window and choose Source->DVD from folder and choose the VIDEO_TS folder.
To play a copied/ripped DVD from the HD with PowerDVD just click on the open button and choose Open DVD File from the Hard Disk Drive and open the video_ts.ifo in the VIDEO_TS folder.
You can rip DVD and convert video file to another format (avi, mpeg, etc.).