Using the WinApi WTSEnumerateServers function you can get a list of all Remote Desktop Servers in a network domain.

Check this code. Tested on Delphi 2007, Delphi 2010, Delphi XE – (Windows XP/7/2008 Server)

program GetRemoteDesktops;

{$APPTYPE CONSOLE}

type
PWTS_SERVER_INFO = ^WTS_SERVER_INFO;
_WTS_SERVER_INFO = packed record
pServerName:LPTSTR;
end;
WTS_SERVER_INFO = _WTS_SERVER_INFO;
WTS_SERVER_INFO_Array  = Array [0..0] of WTS_SERVER_INFO;
PWTS_SERVER_INFO_Array =^WTS_SERVER_INFO_Array;

{$IFDEF UNICODE}
function WTSEnumerateServers( pDomainName: LPTSTR; Reserved: DWORD; Version: DWORD; ppServerInfo: PWTS_SERVER_INFO; pCount: PDWORD):BOOLEAN; stdcall; external 'wtsapi32.dll'  name 'WTSEnumerateServersW';
{$ELSE}
function WTSEnumerateServers( pDomainName: LPTSTR; Reserved: DWORD; Version: DWORD; ppServerInfo: PWTS_SERVER_INFO; pCount: PDWORD):BOOLEAN; stdcall; external 'wtsapi32.dll'  name 'WTSEnumerateServersA';
{$ENDIF}
procedure WTSFreeMemory(pMemory:Pointer);stdcall; external 'wtsapi32.dll' name 'WTSFreeMemory';

procedure GetRemoteDesktopsList(const Domain:PChar;const Servers:TStrings);
var
ppServerInfo : PWTS_SERVER_INFO_Array;//PWTS_SERVER_INFO;
pCount       : DWORD;
i            : integer;
begin
  Servers.Clear;
  ppServerInfo:=nil;
  try
    if WTSEnumerateServers(Domain,0,1,PWTS_SERVER_INFO(@ppServerInfo),@pCount) then
      for i := 0 to pCount - 1 do
        Servers.Add(ppServerInfo^[i].pServerName)
    else
    Raise Exception.Create(SysErrorMessage(GetLastError));
  finally
    if ppServerInfo<>nil then
    WTSFreeMemory(ppServerInfo);
  end;
end;