{ Getting the Windows Directory }

function GetWinDir: string;
var
dir: array [0..MAX_PATH] of Char;
begin
GetWindowsDirectory(dir, MAX_PATH);
Result := StrPas(dir);
end;

// or:

function WindowsDirectory: string;
var
WinDir: PChar;
begin
WinDir := StrAlloc(MAX_PATH);
GetWindowsDirectory(WinDir, MAX_PATH);
Result := string(WinDir);
if Result[Length(Result)] <> '\' then
Result := Result + '\';
StrDispose(WinDir);
end;

// or:

function GetWindowsDirectory(var S: String): Boolean;
var
Len: Integer;
begin
Len := Windows.GetWindowsDirectory(nil, 0);
if Len > 0 then
begin
SetLength(S, Len);
Len := Windows.GetWindowsDirectory(PChar(S), Len);
SetLength(S, Len);
Result := Len > 0;
end else
Result := False;
end;

{ Getting the System Directory }

function SystemDir: string;
var
dir: array [0..MAX_PATH] of Char;
begin
GetSystemDirectory(dir, MAX_PATH);
Result := StrPas(dir);
end;

// or:

function SystemDirectory: string;
var
SysDir: PChar;
begin
SysDir := StrAlloc(MAX_PATH);
GetSystemDirectory(SysDir, MAX_PATH);
Result := string(SysDir);
if Result[Length(Result)] <> '\' then
Result := Result + '\';
StrDispose(SysDir);
end;

// or:

function GetSystemDirectory(var S: String): Boolean;
var
Len: Integer;
begin
Len := Windows.GetSystemDirectory(nil, 0);
if Len > 0 then
begin
SetLength(S, Len);
Len := Windows.GetSystemDirectory(PChar(S), Len);
SetLength(S, Len);
Result := Len > 0;
end else
Result := False;
end;

{ Getting the Temporary Directory }

function GetTempDir: string;
var
Buffer: array[0..MAX_PATH] of Char;
begin
GetTempPath(SizeOf(Buffer) - 1, Buffer);
Result := StrPas(Buffer);
end;

// or:

function GetTempPath: string;
var
TmpDir: PChar;
begin
TmpDir := StrAlloc(MAX_PATH);
GetTempPath(TmpDir, MAX_PATH);
Result := string(TmpDir);
if Result[Length(Result)] <> '\' then
Result := Result + '\';
StrDispose(TmpDir);
end;

// or:

function GetTempPath(var S: String): Boolean;
var
Len: Integer;
begin
Len := Windows.GetTempPath(0, nil);
if Len > 0 then
begin
SetLength(S, Len);
Len := Windows.GetTempPath(Len, PChar(S));
SetLength(S, Len);
Result := Len > 0;
end else
Result := False;
end;

// ****************************

// Example Calls:

procedure TForm1.Button1Click(Sender: TObject);
begin
label1.Caption := GetWinDir;
label2.Caption := GetSysDir;
label3.Caption := GetTempDir;
end;