Yes, but I would not think about it as code sharing. MSI (the installation technology used by Office and most Windows programs) supports the notion of "Shared Components," which allows applications to share common resources. A component is a discrete installation unit, and in the case of large products such as the Office suite, many components make up any one product (Word, Excel, etc.). Rather than sharing actual source code or bundling the same DLLs with each Office product, components are shared between the products. You can envision that rather than each product containing the same code used to generate tables, they all use the same piece of code when generating tables. There are a variety of advantages, of which perfect interop between pieces of software is by far the greatest.

More info on shared components: http://blogs.msdn.com/b/heaths/archive/2009/12/21/about-shar...