1) Open a command prompt
2) Browse to SetActionAccount.exe which should be located in C:\Program Files\Microsoft Forefront\Client Security\Server\Microsoft Operations Manager 2005
3) Type SetActionAccount.exe ManagementGroupName -query  (amend ManagementGroupName as required, default is usually forefrontclientsecurity).

Once you know which account is the MOM action account, ensure it has the correct permissions, such as needs to be part of local admins etc