Got a lot (20+) of this sort of error in my event log.
There is a guide on the Microsoft Wiki abou this here: http://social.technet.microsoft.com/wiki/contents/articles/1916.how-to-rebuild-the-report-server-performance-counters-ssrs.aspx, however, this alone is not enough to fix the issue.
The commands that I ran in order to get mine back up and running, from an elevated command prompt were:
C:\Windows\Microsoft.NET\Framework\v4.0.30319\installutil.exe /u C:\Program Files\Microsoft SQL Server\MSRS10_50.MSSQLSERVER\Reporting Services\ReportServer\bin/ReportingServiceLibrary.dll
C:\Windows\Microsoft.NET\Framework\v4.0.30319\installutil.exe C:\Program Files\Microsoft SQL Server\MSRS10_50.MSSQLSERVER\Reporting Services\ReportServer\bin/ReportingServiceLibrary.dll
net stop ReportServer
net start ReportServer
The errors no longer occurred when rebooting the server after this point.