OK, thanks for the replies.
I am always logged in as administrator since this computer doesn't go online.
I just noticed also that there seems to be a problem with Windows "Programs and features" and Sonar.
While checking if it was possible to uninstall just portions of Sonar, Windows says it is already uninstalled.
So I will have to eventually delete X3 files instead of uninstalling anyway, I guess.
Don't know what happened there, I have never fiddled around with this installation or anything.
I'll just go ahead and install wherever Platinum wants to install.
Thanks again,
Dan