Date: 05/19/06 (Code WTF)    Keywords: no keywords

function IsWin2000: Boolean;
var OS: TOSVersionInfo;
begin
  ZeroMemory(@OS, SizeOf(OS));
  OS.dwOSVersionInfoSize := SizeOf(OS);
  GetVersionEx(OS);
  result := (OS.dwMajorVersion >= 5) and (OS.dwPlatformId =
VER_PLATFORM_WIN32_NT);
end;

function IsWinXP: Boolean;
var OS: TOSVersionInfo;
begin
  ZeroMemory(@OS, SizeOf(OS));
  OS.dwOSVersionInfoSize := SizeOf(OS);
  GetVersionEx(OS);
  result := (OS.dwMajorVersion >= 5) and (OS.dwMinorVersion >= 1) and
(OS.dwPlatformId = VER_PLATFORM_WIN32_NT);
end; 

procedure CheckOSVersion;
Var
GETOS : String;
GETOSMajor : String;
begin
  if IsWinXP = TRUE then begin
    GETOS := 'Windows XP';
    GETOSMajor := 'Windows NTx';
  end
  else
    if IsWinMe = TRUE then begin
      GETOS := 'Windows ME';
      GETOSMajor := 'Windows 9x';
    end
    else
      if IsWin98se = TRUE then begin
        GETOS := 'Windows 98SE';
        GETOSMajor := 'Windows 9x';
      end
      else
        if IsWin98 = TRUE then begin
          GETOS := 'Windows 98';
          GETOSMajor := 'Windows 9x';
        end
        else
          if IsWin95OSR2 = TRUE then begin
            GETOS := 'Windows 95B';
            GETOSMajor := 'Windows 9x';
          end
          else
            if IsWin95 = TRUE then begin
              GETOS := 'Windows 95A';
              GETOSMajor := 'Windows 9x';
            end
            else
              if IsWinNT = TRUE then begin
                GETOSMajor := 'Windows NTx';
                GETOS := GetNTType;
              end; 

Source: http://community.livejournal.com/code_wtf/38548.html