Recently I have been enjoying using the new Powerpivot addin for Excel.  I had to rebuild my machine and I apparently overlooked one small issue.  After reinstalling Office 2010, I launched the powerpivot msi to install.  Everything completed just fine, but to my dismay nothing worked. 

Ron Pihlgren managed to save the day!  Turns out you need to have the Shared Features installed so that you get the VSTO runtime installed so powerpivot will run.  His full post is here.