Just before Christmas I experienced a fileserver cluster running on Windows Server 2008 with poor performance. Researching the problem we figured out that the antivirus was causing the problem. A quick search on Internet gave us a lot of results from other people suffering the same problem.Today I had a customer with the exact same problem - same configuration, same problem.
Applying the latest update from Symantec should fix the problem. ( I think this is update is called R4)
UPDATE: The update did not fix the problems either. Symantec Endpoint Protection is still creating a performance problem on WIndows Server 2008 File clusters. Solution at customer was to install Forefront Client Protection! Way to go (and thanks to Terje for pointing this out :) )