Handbrake or
MPEG Streamclip +
QuickTime MPEG-2 Playback Component will convert DVD video to something Powerpoint might accept. Inform yourself what video formats (.mov or .avi or .mp4) and codecs (H264, Divx, Xvid, DV, Indeo, ...) Powerpoint accepts.
Handbrake is for .avi and .mp4 files using the Xvid and H264 codecs.