Are you working in a composition with non-square pixels? Press "Cntl+K" and look at the Pixel Aspect Ratio, unless it's set to square(how pixels are displayed on a computer), shapes, objects can look squished and distorted. When broadcast - they will look normal.
Below the comp window is a toggle pixel aspect correction button, you can check that and AE will approximate how your video will look as a final. Things may look jagged & rough, but it's just cause it's stretching/squishing the image a bit, won't affect the final render. FYI-if you are working in HD or only for the web, you should always be working with square pixels.
Johnny Cuevas, Editor
"I have not failed 700 times. I have succeeded in proving that those 700 ways will not work. When I have eliminated the ways that will not work, I will find the way that will work."
---THOMAS EDISON on inventing the light bulb.