Tim Newton from the performance team at Microsoft has written 2 really good articles on how to monitor and tune performance on Windows Server 2008 R2. He goes into counters, CPU, memory, pagefiles etc etc. Definitely worth a read

http://blogs.technet.com/b/askperf/archive/2010/08/06/performance-tuning-windows-server-2008-r2-pt-1.aspx

http://blogs.technet.com/b/askperf/archive/2010/11/05/performance-tuning-windows-server-2008-r2-pt-2.aspx