Nero Vision 4 Problem

What could be the reason(s) for the inability of my Nero Vision 4 DVD burning programme to burn a movie file (mpeg-4 or mpeg 1 usually) without loss of quality ? Before burning I check out the quality of the movie and if it’s fine then I proceed with the burn. But every single time after burning the quality has suffered. There is a distinct blurring or lagging when a character moves faster then walking speed. Also during camera-panning there is a stickiness or juddering of the camera motion when it should be smooth and seamless.
My OS is Windows XP Home.
The size of the files are usually between 500MB and 1.5GB.
I normally drag-n-drop the file and then burn, so what’s the problem ?
In layman’s terms please.