Actually, a lot of Microsoft games have a protected installer. This means that the setup.exe (or install.exe or whatever) checks the disc for safedisc protection, and if anything is wrong, refuses to install. I think by adding the text file, you disrupted the copy protection. I'm not sure about this, but Microsoft tends to have protected installers. Maybe you could use
cdkiller's protection scan utility on it, it can tell you what protection it has. If it's safedisc, there are programs to unpack the setup.exe so it will install from a changed media.