[dec]

Hola,

Hace tiempo que ocupo el siguiente código, a ver si te sirve a ti también:

Código Delphi [-]

uses
  Windows;

const
 SECURITY_NT_AUTHORITY : TSIDIdentifierAuthority = ( Value : ( 0, 0, 0, 0, 0, 5 ) );
 SECURITY_BUILTIN_DOMAIN_RID = $00000020;
 DOMAIN_ALIAS_RID_ADMINS = $00000220;
 DOMAIN_ALIAS_RID_USERS = $00000221;
 DOMAIN_ALIAS_RID_GUESTS = $00000222;
 DOMAIN_ALIAS_RID_POWER_USERS = $00000223;

function CheckTokenMembership(TokenHandle: THandle; SidToCheck: PSID; var IsMember: BOOL): BOOL; stdcall; external advapi32;

// http://stackoverflow.com/questions/6...dministrator-r
function IsUserAdmin : Boolean;
var
  b: BOOL;
  AdministratorsGroup: PSID;
begin
  {
    This function returns true if you are currently running with admin privileges.
    In Vista and later, if you are non-elevated, this function will return false 
    (you are not running with administrative privileges).
    If you *are* running elevated, then IsUserAdmin will return true, as you are 
    running with admin privileges.

    Windows provides this similar function in Shell32.IsUserAnAdmin. 
    But the function is deprecated, and this code is lifted
    from the docs for CheckTokenMembership:
      http://msdn.microsoft.com/en-us/library/aa376389.aspx
  }

  {
    Routine Description: This routine returns TRUE if the callers
    process is a member of the Administrators local group. Caller is NOT
    expected to be impersonating anyone and is expected to be able to
    open its own process and process token.
      Arguments: None.
      Return Value:
        TRUE - Caller has Administrators local group.
        FALSE - Caller does not have Administrators local group.
  }
  b := AllocateAndInitializeSid(
      SECURITY_NT_AUTHORITY,
      2, //2 sub-authorities
      SECURITY_BUILTIN_DOMAIN_RID,  //sub-authority 0
      DOMAIN_ALIAS_RID_ADMINS,      //sub-authority 1
      0, 0, 0, 0, 0, 0,             //sub-authorities 2-7 not passed
      AdministratorsGroup);
  if (b) then
  begin
    if not CheckTokenMembership(0, AdministratorsGroup, b) then
      b := False;
      FreeSid(AdministratorsGroup);
  end;

  Result := b;
end;

Ejemplo de uso:

Código Delphi [-]
if IsUserAdmin() then
begin
  // Whatever
end;

En el hilo de Stackoverflow donde lo encontré puedes acaso obtener más información.