Before you produce your final video that you want to place on the web, choose the settings that will make it best for use on the web:
View tutorial on choosing from expert settings in Apple's IMovie
View Windows Movie Maker Tutorial
Another Strategy:
Produce the highest quality settings for the video production. This is sometimes described as "for playing on my computer" (but not "playing on the network").
Put the finished / produced file in a folder of the same name.
Now use a video converter ("transcoder") to produce the final formats that you need.