EXECUTE permission denied for local system user

I have a NetIQ alert that runs a stored procedure that generates an execute permission denied error each time it runs. The NetIQ agent is running as the local system user. As I understand it, the local system user's privileges are equivalent to sysadmin so I don't understand why this error is being created.

 " Error Message: EXECUTE permission denied on object 'monitoring_NetIQ_AllQueues_NLA', database 'wintrack_nla', owner 'dbo'. " , Error Code: 0 , MO Error Code: -2147217911

NT 5.2, SQL 8.00,818,