My video becomes choppy when I save it–what do I do?

My video becomes pixelated and choppy when I save it.  I tried looking at whether my video driver is outdated, don't understand another thread's instructions on looking for codecs, but my computer is fairly new so I don't think it would be outdated.  The videos I'm using are not exorbitantly large.  I tried saving it in the optimal format, the format for e-mail and for the computer but all of them turn out choppy, except when I save it in the Windows Live Movie Maker format.  Can someone please help me on this problem? 

Julie