Delphi Опеределение версии Windows 8, Windows 7, Windows XP, дополненная версия

function WhatWindowsIsInstalled: string;
var
  VerInfo: TOsVersionInfoExA;
begin
  FillChar(VerInfo, sizeof(VerInfo), 0);
  VerInfo.old.dwOSVersionInfoSize := Sizeof(TOsVersionInfoExA);
  if not GetVersionExA(VerInfo.old) then
  begin
    VerInfo.old.dwOSVersionInfoSize := Sizeof(TOsVersionInfoA);
    GetVersionExA(VerInfo.old);
  end;
  case VerInfo.old.dwPlatformId of
    VER_PLATFORM_WIN32_WINDOWS:
      if (Verinfo.old.dwMajorVersion = 4) and
        (Verinfo.old.dwBuildNumber = 950) then
        Result := 'Windows 95'
      else if (Verinfo.old.dwMajorVersion = 4) and
        (Verinfo.old.dwMinorVersion = 10) and
        (Verinfo.old.dwBuildNumber = 1998) then
        Result := 'Windows 98'
      else if (Verinfo.old.dwMinorVersion = 90) then
        Result := 'Windows Me';
    VER_PLATFORM_WIN32_NT:
      if Verinfo.old.dwMajorVersion = 3 then
        Result := 'Windows NT 3.51'
      else if Verinfo.old.dwMajorVersion = 4 then
        Result := 'Windows NT 4.0'
      else if Verinfo.old.dwMajorVersion = 5 then
       begin
        if Verinfo.old.dwMinorVersion = 0 then
          Result := 'Windows 2000'
        else if Verinfo.old.dwMinorVersion = 1 then
          Result := 'Windows XP'
        else if Verinfo.old.dwMinorVersion = 2 then
           Result := 'Windows Server 2003';
       end
      else if Verinfo.old.dwMajorVersion = 6 then
       begin
        if Verinfo.old.dwMinorVersion = 0 then
         begin
          if VerInfo.wProductType=VER_NT_WORKSTATION then
           Result := 'Windows Vista'
            else
          if VerInfo.wProductType<>VER_NT_WORKSTATION then
           Result := 'Windows Server 2008'
         end else
        if Verinfo.old.dwMinorVersion = 1 then
         begin
          if VerInfo.wProductType=VER_NT_WORKSTATION then
           Result := 'Windows 7'
            else
          if VerInfo.wProductType<>VER_NT_WORKSTATION then
           Result := 'Windows Server 2008 R2'
         end else
        if Verinfo.old.dwMinorVersion = 2 then
         begin
          if VerInfo.wProductType=VER_NT_WORKSTATION then
           Result := 'Windows 8'
            else
          if VerInfo.wProductType<>VER_NT_WORKSTATION then
           Result := 'Windows Server 2012'
         end else
        if Verinfo.old.dwMinorVersion = 3 then
         begin
          if VerInfo.wProductType=VER_NT_WORKSTATION then
           Result := 'Windows 8.1'
            else
          if VerInfo.wProductType<>VER_NT_WORKSTATION then
           Result := 'Windows Server 2012 R2'
         end
       end else
         begin
          try
           Result := 'Windows '+IntToStr(Verinfo.old.dwMajorVersion)+'.'+IntToStr(Verinfo.old.dwMinorVersion);
          except
           result:='Empty'
          end;
         end;
    VER_PLATFORM_WIN32s:
      Result := 'Win32s';
  end;
end;

Свежая информация по константам версий: http://msdn.microsoft.com/en-us/library/windows/desktop/ms724834%28v=vs.85%29.aspx
(c)Izhik