What are the differences between Windows EVT and EVTX log files?

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http://serverfault.com – Windows is moving to the *.EVTX format for their logging files, but we still have clients that have versions that write to the older *.EVT format. What are the differences in how those files are built, with a view to if they are parsed differently. The problem I'm having is when I try to open a *. EVT file in windows 7, I get a lot of invalid data errors, but when I turn around and open them in XP, it seems to work fine. For full disclosure, I'm seeing the same behaviour when I save the log out of the viewer, and when I try copying the files directly from the windows32 folder path. (HowTos)