1) If there's a codec problem, then it's most likely you screwed it up. You need to uninstall all codecs first before installing new ones. That's a general wisdom.

2) Shareaza does not trick around with your videos. It's only meant to transfer universal data. If the problem would occur by the networks Shareaza use, then other programs like Morpheus or Gnucleus, would have the same problems.
Shareaza makes a new file to save the preview in, so it doesn't mess around with the real file you're downloading.

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