The downloadable VSTO 2005 SE contains only Visual Studio templates and support for VSTO Add-ins. The full version of VSTO is required in order to create Word and Excel document-level customizations. For VS 2005, this is part of the "Team Suite" edition or must be (have been) purchased as a separate product. The document-level customizations in VSTO 2005 targeted office 2003, primarily.

VSTO 2008, on the other hand, is available as part of VS 2008 Professional and "higher". It contains everything that was in VSTO 2005, plus SE, and also has document-level customization design-time support for Office 2007, as well as support for ClickOnce deployment (for Office 2007 projects).

If you want to target Office 2007 workbook customizations then you need to purchase Visual Studio 2008 Pro (or "higher").

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Cindy Meister, VSTO/Word MVP