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AVI is actually a container format, chances are you probably don't have the correct codec. An AVI could be encoded using anything ranging from MPEG-2 to Xvid.
It's possible to download these online, but as others have suggested it's far easier to just use something like VLC which have the codec support built in.
We are aware of an issue with emails from the Forum to Microsoft-based email accounts (hotmail/outlook/live.com email addresses). This is being looked into currently, thanks for your patience meanwhile.