[Setup]
InfoBeforeFile=C:\readme.txt
AppName = sample
AppVersion = 1.0
DefaultDirName = {tmp}

[Files]
Source: c:\readme.txt; DestDir: {app}; AfterInstall: ExtLog(); Flags: recursesubdirs

[Code]
var
    ProgressLabel: TLabel;

procedure ExtLog();
begin
    with WizardForm.ProgressGauge do begin
      ProgressLabel.Caption:=IntToStr((Position-Min)/((Max - Min)/100)) + '%'
      if (Position-Min)/((Max - Min)/100) > 50 then ProgressLabel.Font.Color:= clWhite
    end
end;

procedure InitializeWizard;
begin
    ProgressLabel:=TLabel.Create(WizardForm)
      ProgressLabel.Top:= 4
      ProgressLabel.Left:= 200
      ProgressLabel.Caption:= '0%'
      ProgressLabel.AutoSize:= True
      ProgressLabel.Font.Color:= clBlue
      ProgressLabel.Font.Style:= [fsBold]
      ProgressLabel.Transparent:= True
      ProgressLabel.Parent:= WizardForm.ProgressGauge
end;