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

[Run]
Filename: "{app}\Utility.exe"; Parameters: "/x"; AfterInstall: AddRegistry;

[Code]
procedure AddRegistry;
begin
  RegWriteDWordValue(HKCU, 'Software\My Company\My Program', 'ShowToolbar', 1);
end;