The newer disk version 1.11.x.x installs into the correct directory in vista automatically, but not the older versions like 1.8.x.x As such pls try to install the main disk, the addons and patch in vista using the procedure which you can download here http://www.aerofly.com/ipacs_vista.pdf Disable the UAC too

One more thing is that since you're installing Addon 5, there is no need to install any patch anymore. So, just install the main disk and addons in sequence to the correct directory as per the instructions