If you receive following errors in the Operations Manager Event Log:

 

Event ID 455 

Description: HealthService (36900) Health Service Store: Error -1032 (0xfffffbf8) occurred while opening logfile C:\Program Files\System Center Operations Manager\Agent\Health Service State\Health Service Store\edb.log.

Event ID 489

Description: HealthService (36900) Health Service Store: An attempt to open the file "C:\Program Files\System Center Operations Manager\Agent\Health Service State\Health Service Store\edb.log" for read only access failed with system error 32 (0x00000020): "The process cannot access the file because it is being used by another process. ".  The open file operation will fail with error -1032 (0xfffffbf8).

and have set the antivirus exclusion list as per http://support.microsoft.com/kb/975931

most probably the process is still being locked by the antivirus software. The quickest way would be a reboot. If this is not possible, you can wait for a couple of hours that the process unlocks itself or use an unlocker 3rd party software like http://emcosoftware.com/unlock-it