History of this question
Because I had tried so many ways to get Windows on this 20" iMac 9,1, I didn't want to come here and write a book about it. I had found a way to do it myself, install the Blue Tooth drivers and run my solar system inverter management software successfully, which had already solved my key problem.
My original intention with this question was to ask for information about the Windows Support drivers, not to ask about installing Windows. The first answer here gave me advice about installing Windows on this machine, that answer is appreciated and has got me sucked in, to understand why those earlier attempts failed. So, I've rewritten the question to focus on solving that problem.
Background to the question
When I first set out to install Windows on my 20" iMac 9,1, running MacOS 10.9.5, I ran the Boot Camp assistant, which downloaded the Windows support software and that is the only thing that seemed to work. It said I needed "a single full install Windows installation disc". I had purchased a licensed full install DVD for 64 bit Windows 7 Home Premium with Service Pack 1 some time ago and then later on purchased an upgrade licensed DVD for Windows 8 Pro. I had inserted the Windows 7 DVD but the Boot Camp assistant complained it couldn't find my install disc which was clearly mounted. There were no other options available from the assistant.
I then did some extensive online research and tried all kinds of methods to get Boot Camp assistant to work, to no avail. So, I created a Windows partition myself, rebooted from the Windows 7 install disc, chose the format option and let the installer do its thing. When I tried to run setup.exe from the BootCamp folder it said my computer was not supported. So I then ran the Windows 8 upgrade from within Windows 7 (because the license I bought was only for an upgrade). I then allowed Windows 8 to upgrade, which was necessary before the Windows Store would provide the free 8.1 upgrade, which I then installed. I again tried to run setup.exe from the BootCamp folder, with the same result as before.
An idea I got online at some point a while ago was to run BootCamp64.msi from the command prompt running as an Administrator. This also failed. So then I just installed the BlueTooth and the RealTek Sound drivers myself and that worked fine. So, the only thing Boot Camp assistant did for me is download the Windows support software. I did everything else myself.
My current situation works for me, I have Windows 8.1 running and my inverter software works fine and I can download and store all the data on DropBox, which also installed and works fine so I can use other apps on other machines to analyse it. I can also organise all the data in Excel on this iMac, as Microsoft Office for Windows also installed without a hitch.
But, I would like to figure out why Boot Camp would not work for me.
An odd comparison
The 20" iMac 9,1 became free recently. It takes up less space and so I'm using it solely for my solar system. I have a 24" iMac 9,1 with Snow Leopard running on the Mac side because of some very large music projects created using Cubase 3 back in 2009-10, which I am now in the process of recovering and incorporating into Logic Pro X. I've had 64 bit Windows 10 running on that machine for some time now. I started that process in 2012, soon after having our solar system installed, running the Snow Leopard Boot Camp assistant with the same 64 bit Windows 7 install disk.
My memory on this is not brilliant, because I was impatient to get it to work and didn't document my steps and just attacked it until it worked. I'm pretty sure Boot Camp assistant worked fine up to the point of running Boot Camp's setup.exe, which complained that this computer was not supported. But I quickly discovered, when poking around inside the Boot Camp folder, that I could run BootCamp64.msi and then the Boot Camp control panel and all the appropriate drivers installed appropriately. There was also no issue with Apple Software Update and the installation of iTunes and the setup of iCloud. Then the free Windows 10 update offer came along which I accepted immediately and that's been running well and updating appropriately ever since along with the Boot Camp control panel and Apple Software Update. And I really like the way my Safari favourites work seamlessly with Firefox.
Hence my initial confidence when I set out to do this on the 20" iMac 9,1. This has been going on for more than a week now during which I've started from scratch several times and I'm now developing some serious PTSD towards the whole thing. But if all aspects of Boot Camp run on the other one, why not this one? It's obvious to guess it might have worked on the other computer because I started with Snow Leopard, but the complaints about compatibility occur while Windows is running. Maybe the Boot Camp 4 executables have been changed in the intervening six years?