VSTO 2005 Redistributable Package
Another intresting download: Microsoft Visual Studio 2005 Tools for the Microsoft Office System (VSTO 2005) Redistributable Package (x86)
VSTO 2005 is the technology that brings the power and productivity of Visual Studio .NET and the .NET Framework to business solutions built on Microsoft Word 2003, Microsoft Outlook 2003, Microsoft Excel 2003 or Microsoft InfoPath 2003. Download here