Installing and Configuring PowerPivot has been painful for me – Partly because I managed to dive in and not read the readme.txt and release notes 😉
The three things I did wrong and have learned are:
– “Standalone” SharePoint option is not supported as this uses network service accounts. (see below why)
– Domain accounts are a MUST. No local option (yet)
– You need to login with the service account in “Interactive” MODE.
More tips are on http://powerpivotgeek.com/server-installation/reported-problems/#001 thanks to Dave Wickert.