The error usually goes something like this...
Error: Could not load file or assembly 'Microsoft.AnalysisServices.SharePoint.Integration, Version=10.0.0.0, Culture=neutral, PublicKeyToken=89845dcd8080cc91' or one of its dependencies. The system cannot find the file specified.
Open up command prompt with elevated permissions and run this command
RD /S C:\Windows\Assembly\GAC_MSIL\Microsoft.AnalysisServices.SharePoint.Integration
In most cases you will get an Access Denied error. That is because w3wp.exe is holding onto the file. You will need to stop IIS and run the command again.
1. After the installer fails disable the assembly cache viewer.
2. Go into the assembly and find the file Microsoft.AnalysisServices.SharePoint.Integration.dll
3. Copy this dll to the bootstrap location: C:\Program Files\Microsoft SQL Server\100\Setup Bootstrap\SQLServer2008R2\x64
4. Remove the Powerpivot instance from the Program and Features .. SQL Server 2008 R2 Uninstall
5. Enable the assembly cache viewer and copy the Microsoft.AnalysisServices.SharePoint.Integration.dll into the GAC.
6. Run the installation process again to install the PowerPivot instance.