ok, this one has burned some of my time a couple times now.
When you are configuring Enterprise Manager in a Windows environment you need to modify the Windows Local Security Policy to "Allow Logon as Batch" for whatever user is running the Oracle DB Console service. Otherwise, you will be unable to assign preferred credentials and thus will be... ok, this one has burned some of my time a couple times now.
When you are configuring Enterprise Manager in a Windows environment you need to modify the Windows Local Security Policy to "Allow Logon as Batch" for whatever user is running the Oracle DB Console service. Otherwise, you will be unable to assign preferred credentials and thus will be unable to create RMAN backups on the server.

Read more http://billennis-ssts.blogspot.com/2011/04/oracle-on-windows-logon-as-batch-needed.html