SQL 2008 R2 Repair: Setup failed to install Windows PowerShell. Error code is -2145124329
Well, that error code is:


  SUS_E_NOT_APPLICABLE -- install is not needed because no updates are
  applicable


(source)

This happens when you're trying to install a patch for an application and the patch no longer applies to the application.  For example, if the application is version 10 and the patch fixes version 9, the patch will fail with that error.

One possible fix is to uninstall and reinstall SQL Server.  Also, you could check to see if you have any patches or fixes installed already.  I know that Service Pack 1 is already out for SQL Server.  If you have that installed, it could be causing this error (trying to add an old patch to a newer version of PowerShell).