The order in which you install SQL Server 2008 R2, SharePoint 2010, and PowerPivot for SharePoint on a single server currently matters. There is a glitch that occurs when you install some components of SQL Server 2008 R2 prior to installing PowerPivot.
This is the order that works for me:
- Provision a new Windows 2008 SP2 server.
- Add IIS role.
- Make it a domain controller with dcpromo.
- Install SharePoint 2010 prerequisites.
- Install SharePoint 2010, do not check configure at the end.
- Install PowerPivot instance (choose new server) from SQL Server 2008 R2 installation.
Now you may install any other instances of SQL Server 2008 R2.
Here is a link that helped me identify the problem: