How to get a clean install of Windows 10 to activate?

I have Windows 8.1 Pro 64 bit installed on my iMac (20-inch mid 2007). I have downloaded from Microsoft a free copy of Windows 10 Pro 64 bit. Microsoft did not ask me to do so. I decided since Window 10 is free, I would like to try it. I do not have a Product Key. During installation, I want to format my Windows partition and do a clean install of Windows 10. When finished, I what Windows 10 to be activated.

Update 1:

In response to bmike answer, I will add the following.

My Mac's firmware will not boot the 64 bit Windows DVD releases by Microsoft. I have to burn a special DVD where the EFI booting is removed. The procedure is given here. I also have problems installing the Boot Camp Support Software 4.0.4033. The solution to this problem is posted here. So when Apple posts, basically you have to have a 2012 or newer Mac to run Windows 10, I would not believe this to be true.

Microsoft has always incorporated the ability to use previous version drivers to run Windows. This goes all the way back to the days of OS/2.

Solution 1:

According to Apple's Knowledge Base article, your iMac isn't supported directly in Boot Camp.

• https://support.apple.com/kb/HT204990

You should probably get VirtualBox or another VM solution to run the Windows 10 as that would bridge the driver gap better than trying to piece things together yourself. It might be possible, but I don't know where you'd start on it...

Solution 2:

I upgraded to from Windows 8.1 to Windows 10. I was able to either click "Skip" or "Do this later" when asked for the Product Key. Once I finished installing, Windows 10 activated.

Running Windows 10 from an upgrade was a disaster. However, this fixed my problem. Now, when I do clean installs, Windows 10 activates.