SQL Server 2008 R2 64bits repair error
This is a known problem with this particular DLL during the repair process.  Copying and renaming the file seems to be a suggested work around - see:

http://connect.microsoft.com/SQLServer/feedback/details/576412/-the-process-cannot-access-the-file-c-windows-syswow64-perf-mssql10-50-mssqlserver-sqlagtctr-dll-because-it-is-being-used-by-another-process