Here are some of the important steps to consider while installing VSTA SDK (as observed by our Summer Intern, Harish):

The initial step is to watch the Intro video http://go.microsoft.com/?linkid=6676089, which gives a quick overview of the basics of VSTA. But does not provide us with enough information about setting up VSTA with pre-installed VS 2005.

After downloading and running 'vsta_sdk.msi', the steps for setting up VSTA with VS 2005 are in a ReadMe file located in a folder :"%SYSTEMDRIVE%\Program Files\Visual Studio 2005 Tools for Applications SDK\2007.02\VisualStudioToolsForApplications".

The two steps described are quite straight forward. But these are the most important steps... especially running the installer file 'vsta_aide.msi' with the command-line parameters "VSTAINVS = 1". This parameter is very important for the VSTA installation and setting up VSTA with VS 2005.

To verify, open the IDE for VS 2005 -> go to "Help" menu and click "About Microsoft Visual Studio". There should be an entry of VSTA as "Microsoft Visual Studio 2005 Tools for Applications" under 'Installed Products'. If that entry is not there then uninstall the SDK and reinstall OR Repair the installation by running "vsta_sdk.msi" again. And then repeat the step of running installer file 'vsta_aide.msi' with command-line parameters "VSTAINVS = 1".

Above all, the best location to start with VSTA (after the SDK installation) is to refer VS 2005 IDE's "Help" menu -> click Contents -> then go down to "Visual Studio 2005 Tools for Applications SDK" in the left pane. It's the most helpful resource (from those I have used) for any VSTA Beginner-level programmer...

Summit has created some excellent micro-sample integrations that are smaller and as such easier to understand than the SDK samples.

For C# managed applications: TestCon - Minimal sample VSTA integration

For C++ COM applications (MFC, ATL): COMClient easy integration - using generic COMProxy and VAO. This sample's approach is also compatible for VBA side-by-side integrations.

For VB6 applications: VB6 COMClient Easy Integration.  This sample's approach is also compatible for VBA side-by-side integrations.  

All of these samples show VSTA integrations using Summit's assembly/sample code, VSTA Access Objects (VAO) Source

VAO contains similar integration source to the SDK samples, but it is consolidated into one place for ease of use.  VAO also implements VBA-Style "seamless, non-destructive debugging.