Faire une mise à jour d'un fichier binaire

procedure TForm1.Button1Click(Sender: TObject);
var
f: file;
l: longint;
datei, altstring, neustring, s: string;

begin
altstring := 'Démarrage de Windows 95...';
neustring := 'Fermeture de Windows 95... ';
datei := 'C:Copie de io.sys';

s := altstring;
assignFile(f, datei);
reset(f,1);
for l := 0 to filesize(f)-length(altstring)-1 do
begin
Application.ProcessMessages;
seek(f,l);
blockread(f,altstring[1],length(altstring));
if altstring = s then
begin
seek(f,l);
blockwrite(f,neustring[1],length(neustring));
label1.caption := 'Status: Stelle gefunden und gepatcht!';
end;
Application.ProcessMessages;
end;
closeFile(f);
end;