I got a reply...
When MS released FSX, they made an error in their installation that causes the registry entries in FS9 to corrupt. Since, like most all products that are installed using industry standard installation tools- PMDG's products look to the registry for current information on your FS9 installation- this corruption creates a problem.
The solution to this problem is pinned prominently at the top of our customer support forum. I'll copy it below for your simplicity:
Captains-
As more of you are finding copies of FSX at your local software store- we're getting a few reports that in some instances FSX may delete some registry information related to FS9.
We've been working with FSX for some time and find this not to be a normal condition- but it seems that a few of you are running into this- so I thought we'd post a notam here to help you out.
The problem is essentially thus: All PMDG installer check for FS9.1 before installation. If the FS9.1 installation path is not present- then the installer will tell you it cannot run. The only way to fix this is to correct your registry.
Another user this morning pointed out that our friends over at Flight1 have a useful freeware tool that will fix this for you- so rather than re-invent the wheel or tell you how to do it manually- I'd recommend using their tool instead:
http://www.flight1.com/view.asp?page=library
Run this tool- then you SHOULD be okay with your FS9.1 installs.... And pass your thanks for this tool along to Flight1.
Regards,
Paul Gollnick
Precision Manuals Development Group
Manager Customer and Technical Support
www.precisionmanuals.com
Lets pray it works!
Have to go school 😞
I'll try it L8R and report back 😎