Hey Gary,
Well I wouldn't really know any of that

I also have msxml 3 and 6 installed, together with 4, but I think the problem is that, for some reason, even if you have msxml 4 installed previously to install the ATC SDK, the installation of said SDK 'overwrites' or changes something in the msxml 4 installation. And for some reason, that 'something' makes it so that it can't be overwritten or deleted when you try to install/uninstall with the original Microsoft MSXML 4 installer

Classic or not, it is indeed hell!

And like arcueil noted in the post I found that helped me; If I who am quite tech savy on a 'superficial' level, but not on a deeper level (code/programming/so on..), had such a hard time figuring and solving this out; how is a person who knows
just enough about computers to use the many scenery design tools for FS expected to figure this out before he/she either nukes their setup, or throws it out the window altogether??

Also just like arcueil said, I hate hate hate hate Windows! I've always used MacOS since my first computer, and only installed using windows for FS and other games!
But I don't wanna start ranting again

Leonardo