Install the KB982307 hotfix as a prerequisite for PowerPivot

NOTE:  This KB hotfix has to be installed on all web front end as well as application servers in your SharePoint farm.

1. Begin by downloading the applicable architecture (x86/x64/i64) version for your environment from here: http://www.microsoft.com/downloads/details.aspx?displaylang=en&FamilyID=3e102d74-37bf-4c1e-9da6-5175644fe22d
2. Login to your server as an administrator.
3. Open Windows Explorer.
4. Browse to the downloaded .msu file and double click it.  In the case of a x64 server, the file name is “Windows6.1-KB982307-x64.msu”.
5. Confirm installation by clicking “Yes”. 
6. Wait while the updates is being installed.
7. Upon completion of the installation, you will need to restart your server.  Click “Restart Now” to complete the installation.

Cheers
C