function GetCPUUsage(var liOldIdleTime, liOldSystemTime: LARGE_INTEGER): string;
type
  TSystem_Basic_Information = packed record
    dwUnknown1: DWORD;
    uKeMaximumIncrement: ULONG;
    uPageSize: ULONG;
    uMmNumberOfPhysicalPages: ULONG;
    uMmLowestPhysicalPage: ULONG;
    uMmHighestPhysicalPage: ULONG;
    uAllocationGranularity: ULONG;
    pLowestUserAddress: Pointer;
    pMmHighestUserAddress: Pointer;
    uKeActiveProcessors: ULONG;
    bKeNumberProcessors: byte;
    bUnknown2: byte;
    wUnknown3: word;
  end;

  TSystem_Performance_Information = packed record
    liIdleTime: LARGE_INTEGER; {LARGE_INTEGER}
    dwSpare: array[0..75] of DWORD;
  end;

  TSystem_Time_Information = packed record
    liKeBootTime: LARGE_INTEGER;
    liKeSystemTime: LARGE_INTEGER;
    liExpTimeZoneBias: LARGE_INTEGER;
    uCurrentTimeZoneId: ULONG;
    dwReserved: DWORD;
  end;

  TNtQuerySystemInformation = function(infoClass: DWORD; buffer: Pointer; bufSize: DWORD; var returnSize: Dword): DWORD; stdcall;
var
  NtQuerySystemInformation: TNtQuerySystemInformation;
  SysBaseInfo: TSystem_Basic_Information;
  SysPerfInfo: TSystem_Performance_Information;
  SysTimeInfo: TSystem_Time_Information;
  status: DWORD; {long}
  dbSystemTime: Double;
  dbIdleTime: Double;
begin
  Result := '';

  NtQuerySystemInformation := GetProcAddress(GetModuleHandle('ntdll.dll'), 'NtQuerySystemInformation');
  if @NtQuerySystemInformation = nil then Exit;
  
  if (NtQuerySystemInformation(0, @SysBaseInfo, SizeOf(SysBaseInfo), status) <> 0)
    or (NtQuerySystemInformation(3, @SysTimeInfo, SizeOf(SysTimeInfo), status) <> 0)
    or (NtQuerySystemInformation(2, @SysPerfInfo, SizeOf(SysPerfInfo), status) <> 0) then Exit;

  if liOldIdleTime.QuadPart <> 0 then
  begin
    // CurrentValue = NewValue - OldValue
    dbIdleTime := Int64(SysPerfInfo.liIdleTime) - Int64(liOldIdleTime);
    dbSystemTime := Int64(SysTimeInfo.liKeSystemTime) - Int64(liOldSystemTime);

    // CurrentCpuIdle = IdleTime / SystemTime
    dbIdleTime := dbIdleTime / dbSystemTime;

    // CurrentCpuUsage% = 100 - (CurrentCpuIdle * 100) / NumberOfProcessors
    dbIdleTime := 100.0 - dbIdleTime * 100.0 / SysBaseInfo.bKeNumberProcessors{ + 0.5};

    // Show Percentage
    Result := FormatFloat('0.00 %',dbIdleTime);
  end;
  liOldIdleTime := SysPerfInfo.liIdleTime;
  liOldSystemTime := SysTimeInfo.liKeSystemTime;
end;