There were numerous changes here are only the more important things.

Version 4.00 ( - CURRENT VERSION - ) (Release date: 2007-04-23)

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      Improved Vista compatibility
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      New display option: Center image in window (Menu: View->Display options)
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      Toolbar skin option added (Properties->Toolbar)
        More Toolbar skins are available on the IrfanView homepage.
        => Users can send new Toolbar skins by email, Thanks guys!
    *
      New Properties->Video option: Use DirectShow for playing (good for MJPG AVIs)
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      New advanced batch option: Auto adjust colors
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      "Enhance colors" can be now used on selections
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      New EXIF dialog button: Show in Google Earth (if GPS data available)
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      The EXIF dialog can stay displayed on screen
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      Thanks to Dmitri Poguliayev for the new Vista icon!
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      New Hotkey: CTRL + SHIFT + M = Show/edit JPG comment
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      Several PlugIns are changed/updated, please install the newest versions
        http://www.irfanview.com/plugins.htm
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      Some bugs fixed (Start folder, RLE BMPs, ANI loading)