if you haven't found an answer yet may this will work.(http://blogs.msdn.com/b/karthick_pk/archive/2012/07/16/the-process-cannot-access-the-file-c-windows-system32-perf-mssql10-50-mssqlserver-sqlagtctr-dll-because-it-is-being-used-by-another-process.aspx)

I have had the same issues with CU2 for SQL Server 2008 R2 SP2. Haven't test the fix yet but will port when I do.

--edit

renaming the file specified in the details.txt file then running the CU installer again worked. CU installation completed successfully.