To produce a display electric beams controlled by electromagnets scan from side to side across the screen, building a TV picture by creating a series of lines. Unfortunately the way in which TVs were first invented meant that the beam couldn't scan quick enough to create a stable image in one pass. To resolve this problem the picture seen on a TV screen is made up of two separate fields, each of which is displayed alternatively. The first field displays the odd lines (1,3,5...) and the second the even lines (2,4,6...) and thus the interlaced display was born. Effectively each frame is shown at half resolution.
However, progressive displays such as PC monitors don't use this method of building a picture; rather all lines are displayed at the same time. This is a problem. Interlaced video plays at 25fps (pal), but is actually made up of 50 frames (25 frames in each field). Watching an interlaced display on a computer screen will play back both fields at the same time even though the second field is one frame ahead of the first (still with me?), resulting in the following
So, we can't playback interlaced video on a PC.