Impossible to even guess without more information!
Start by describing what export settings you're using. Screenshot are always helpful. Here's a composite of 3 screenshots following an export I did earlier today. See if a comparison offers any obvious clues.
Edit: The quality of image displays on this site is poor. Here's a direct link:
What is the resolution of the initial video clips/images you have on the timeline?
If they are less than HD resolution then that is where the pixellation is coming from. As an example a standard DVD (UK) video is 720*576 pixels if you try to make HD at 1920*1080 (UK HD) from this then your are increasing the pixel size by a factor of approx 2.7 horizontally and 1.9 vertically (ie approx 5x the area)
For HD you need HD resolution video or images equal to or greater than the HD resolution you are using.
Pixellation can also come from compressing the video too much, although this is unlikely if you have used the default settings.
Thanks to all who have posted assistance. All answers have been useful in each respect and have now exported videos which appear to be of good quality. Johnbaker - I have chosen HDTV 1920 x 1080 HD264 25 FPS which links in with my Sony Bravia HDTV.