Deinterlacing video is the process of taking interlaced video and outputting video with only one field per frame. There are many software tools to do it, including Virtualdub. There are a few common methods of deinterlacing video.
Blending the fields together is considered to be the method with the least loss of quality. It simply blends the two fields into one field. Its biggest disadvantage is that it will create blurred video if the source video has lots of motion.
Discarding a field removes every line from one field, making the resulting image have half the height of the original.
Field duplicating replaces one of the fields with the other field. The resulting image has the same number of horizontal lines as the original image, but it has half the actual vertical resolution.