Vidding discussion

I have a quick question about Windows Media Player that I'm hoping you all could help with. I made a vid in Windows Movie Maker (Vista, 64-bit) and managed to "publish the movie" with no problems. But though my vid looked fine in the small preview screen of Movie Maker, the quality is low and there are pixellated blocks in some scenes in the first minute of the vid.

I've re-published a few times and the distortions come up in different clips each time but always just at the beginning of it, the quality is quite good later on in the vid. I've even tried dragging the clips out and putting new freshly cut clips in. Doesn't help. Anyone have any ideas?
 
I doubt it's a Codec problem because everything else works just fine it's just a few blocky-color spots and pixellation problems.

Any thoughts would be greatly appreciated!