A Server Operator can perform many of the same tasks that an Administrator can, with the exception of account management.

By virtue of the default permissions on

HKLM\Software\Microsoft\Windows NT\CurrentVersion\Winlogon

a Server Operator could replace the SYSTEM value with an executable that grants administrative priveledges. To quote Microsoft:

                              In the environments where members of server operators are not sufficiently                              trusted, it is recommended that security on following keys be changed as below:                              Registry Key                                    Recommended Permissions                              HKEY_LOCAL_MACHINE\Software\Microsoft                              \Windows NT\CurrentVersion\Winlogon             CREATOR OWNER: Full Control                                                                                      Administrators: Full Control                                                                                      SYSTEM: Full Control                                                                                      Everyone: Read