Starting with Windows Vista , Microsoft introduces the The Software Licensing API (SLAPI), this API can be used to determine a genuine Microsoft Windows installation.

So using the SLIsGenuineLocal function you can check if your app is running in a genuine Windows installation.

This is the definition of the function

HRESULT WINAPI SLIsGenuineLocal(
  __in         const SLID *pAppId,
  __out        SL_GENUINE_STATE *pGenuineState,
  __inout_opt  SL_NONGENUINE_UI_OPTIONS *pUIOptions
);

The use of this funtion is very easy, only you must pass the GUID (Application Id) of Windows {55c92734-d682-4d71-983e-d6ec3f16059f} and a variable of type SL_GENUINE_STATE to receive the status of the license.

Check this delphi implementation

{$APPTYPE CONSOLE}
uses
  Windows,
  SysUtils;

type
  SLID  = TGUID;
  _SL_GENUINE_STATE = (
    SL_GEN_STATE_IS_GENUINE        = 0,
    SL_GEN_STATE_INVALID_LICENSE   = 1,
    SL_GEN_STATE_TAMPERED          = 2,
    SL_GEN_STATE_LAST              = 3
  );
  SL_GENUINE_STATE = _SL_GENUINE_STATE;

function SLIsGenuineLocal(var pAppId: SLID; var pGenuineState: SL_GENUINE_STATE; pUIOptions: Pointer): HRESULT; stdcall; external 'Slwga.dll' name 'SLIsGenuineLocal' delayed;

Var
  pAppId : SLID;
  pGenuineState: SL_GENUINE_STATE;
  Status: HRESULT;
begin
  try
    if Win32MajorVersion>= 6 then //Windows Vista o newer
    begin
      pAppId:=StringToGUID('{55C92734-D682-4D71-983E-D6EC3F16059F}');
      Status:=SLIsGenuineLocal(pAppId, pGenuineState,nil);
      if Succeeded(Status) then
        case pGenuineState of
            SL_GEN_STATE_IS_GENUINE       : Writeln('The installation is genuine.');
            SL_GEN_STATE_INVALID_LICENSE  : Writeln('The application does not have a valid license.');
            SL_GEN_STATE_TAMPERED         : Writeln('The Tampered flag of the license associated with the application is set.');
            SL_GEN_STATE_LAST             : Writeln('The state of the installation has not changed since the last time it was checked.');
        end
      else
        Writeln(SysErrorMessage(Cardinal(Status)));
    end
    else
        Writeln('OS not supported');
  except
    on E: Exception do
      Writeln(E.ClassName, ': ', E.Message);
  end;
  Readln;
end.

In windows XP does not exist the SLAPI, but you can use the Win32_WindowsProductActivation WMI class to get simmilar information. the key is check the ActivationRequired property, If return 1 then the system activation is pending for the system. else If returns 0 (zero) the activation is not required.

{$APPTYPE CONSOLE}

uses
  SysUtils,
  ActiveX,
  ComObj,
  Variants;

procedure  GetWin32_WindowsProductActivationInfo;
const
  WbemUser            ='';
  WbemPassword        ='';
  WbemComputer        ='localhost';
  wbemFlagForwardOnly = $00000020;
var
  FSWbemLocator : OLEVariant;
  FWMIService   : OLEVariant;
  FWbemObjectSet: OLEVariant;
  FWbemObject   : OLEVariant;
  oEnum         : IEnumvariant;
  iValue        : LongWord;
begin;

  if (Win32MajorVersion=5) and (Win32MinorVersion=1) then
  begin
    NullStrictConvert :=False;
    FSWbemLocator := CreateOleObject('WbemScripting.SWbemLocator');
    FWMIService   := FSWbemLocator.ConnectServer(WbemComputer, 'root\CIMV2', WbemUser, WbemPassword);
    FWbemObjectSet:= FWMIService.ExecQuery('SELECT * FROM Win32_WindowsProductActivation','WQL',wbemFlagForwardOnly);
    oEnum         := IUnknown(FWbemObjectSet._NewEnum) as IEnumVariant;
    while oEnum.Next(1, FWbemObject, iValue) = 0 do
    begin
      Writeln(Format('Windows is Activated  %s',[BooltoStr(FWbemObject.ActivationRequired=0,True)]));
      Writeln(Format('ActivationRequired    %d',[Integer(FWbemObject.ActivationRequired)]));
      Writeln(Format('Description           %s',[String(FWbemObject.Description)]));
      Writeln(Format('ProductID             %s',[String(FWbemObject.ProductID)]));
      if FWbemObject.ActivationRequired=1 then
      begin
        Writeln(Format('RemainingEvaluationPeriod    %d',[Integer(FWbemObject.RemainingEvaluationPeriod)]));
        Writeln(Format('RemainingGracePeriod         %d',[Integer(FWbemObject.RemainingGracePeriod)]));
      end;
      Writeln(Format('ServerName            %s',[String(FWbemObject.ServerName)]));
      Writeln(Format('SettingID             %s',[String(FWbemObject.SettingID)]));

      Writeln;
      FWbemObject:=Unassigned;
    end;
  end
  else
  Writeln('OS not supported');
end;


begin
 try
    CoInitialize(nil);
    try
      GetWin32_WindowsProductActivationInfo;
    finally
      CoUninitialize;
    end;
 except
    on E:EOleException do
        Writeln(Format('EOleException %s %x', [E.Message,E.ErrorCode]));
    on E:Exception do
        Writeln(E.Classname, ':', E.Message);
 end;
 Writeln('Press Enter to exit');
 Readln;
end.