[Setup]
AppName=My Program
AppVersion=1.5
DefaultDirName={pf}\My Program

[Code]
var
  EditIndex: Integer;
  InputPage: TInputQueryWizardPage;

procedure MyEditChange(Sender: TObject);
begin
  // enable the next button if the edit box is not empty; disable otherwise
  WizardForm.NextButton.Enabled := InputPage.Edits[EditIndex].Text <> '';
end;

procedure MyEditKeyPress(Sender: TObject; var Key: Char);
var
  KeyCode: Integer;
begin
  KeyCode := Ord(Key);
  if not ((KeyCode = 8) or ((KeyCode >= 48) and (KeyCode <= 57))) then
    Key := #0;
end;

procedure InitializeWizard;
begin
  // create the input query page
  InputPage := CreateInputQueryPage(wpWelcome, 'Caption', 'Description', 'SubCaption');
  // add an edit box and remember its index; False means it is not a password edit
  EditIndex := InputPage.Add('Some value:', False);
  // bind events to the just added edit box
  InputPage.Edits[EditIndex].OnChange := @MyEditChange;
  InputPage.Edits[EditIndex].OnKeyPress := @MyEditKeyPress;
end;

procedure CurPageChanged(CurPageID: Integer);
begin
  // if the currently turned wizard page is the one with the edit box, enable
  // the next button if the edit box is not empty; disable otherwise
  if CurPageID = InputPage.ID then
    WizardForm.NextButton.Enabled := InputPage.Edits[EditIndex].Text <> '';
end;

procedure CurStepChanged(CurStep: TSetupStep);
begin
  // if the installation has just finished, save the edit box text into a file
  if CurStep = ssPostInstall then
    SaveStringToFile('C:\File.txt', InputPage.Edits[EditIndex].Text, False);
end;