ShikamaruNara: Am I the only one who is experiencing this error?
I can't solve your problem, unfortunately, but I can tell you that I bought HoI3 and the HoI3 DLC pack a couple of days ago, and I've installed them (in XP) without any problems.
I guess maybe that's slightly helpful for you. At least you know the installer
should be working.
I hope you get it working. GOG.com is usually pretty good about fixing things like this, so you should submit a support ticket if you haven't done that already.
You picked an interesting time to buy your first GOG.com game. It's a new release here, and I notice the Paradox forum is undergoing a major reconstruction too. But don't give up!