iMac 21.5" Late 2009 with Nvidia 9400 graphics card running Windows 10

Update.

As mentioned before I've tried all the drivers and tweaks available for the Nvidia card but none of them worked. However I have managed to adjust the brightness to a desired level. I booted the iMac into OSX using an external USB drive (I basically connected the old hard disk that I had removed from the iMac with an external USB adaptor and selected that after holding option key at startup). I was then able to adjust the screen brightness to a desired level as well as the sound to the lowest level in OSX and shutdown the iMac. I then restarted without the external drive so it booted into Windows 10 with a low startup chime as well as the screen brightness that I wanted. The sound can now be adjusted from within Windows except the startup chime is not extremely loud. The brightness cannot be adjusted however but stays at the level set from within OSX. If the nvram is reset by holding the option+command+P+R key then it all goes back to max; sound as well as brightness.

This is of course not a permanent solution but is the only option I've found until something better comes up.

I've downloaded various versions of 9400 as well as 9400M drivers but none of them worked.

The bootcamp 5.x drivers fixed all other issues in windows 10 except coprocessor driver apart from of course the graphics card driver but there is the smu driver in the bootcamp 4.x which sorts out the coprocessor so only problem is the Nvidia 9400 which was there before I installed any bootcamp drivers.

To be honest rather than installing windows 7 and upgrading I would install High sierra and try bootcamp or just live with the full brightness until a solution comes up. :)

Just to mention that after the SSD upgrade this 10 years old iMac is almost as fast as my recent model Dell with a 6th gen i7 CPU. I have 8GB of RAM in the iMac also so probably that helps somewhat.

Yes I heard about that at some point but this iMac did not suffer any BSOD and I had the latest Windows 10 update (1909) installed prior to the SSD upgrade. I have been able to upgrade to the latest version now as well without any other issues as the USB I used to install Windows initially on this SSD was the same as the one I used previously. Only I had to use the latest Windows 10 ios to upgrade as the auto update would cause the imac to crash during installation most likely due to it installing nvidia driver during the upgrade process. Machaldriver is indeed used for the bootcamp application but as I do not have any OSX partition on the SSD except for the small 200MB efi system partition that was created by the OSX disk utility before windows was installed, bootcamp application doesn't work since it has no OSX partition to look at and select in order to start the iMac with OSX so can actually be uninstalled once all the necessary drivers are installed. I did try and use the newer versions of Machaldriver.sys but it didn't make any difference to the nvidia driver issue or the brightness control.

Yes the first crash was after windows 10 installed the graphic card driver and rebooted the computer. After the reboot and a couple of crashes windows reverted to the windows basic display driver. I installed the bootcamp drivers later as a number of drivers were missing in device manager. I also attempted to install the display driver in safe mode but on rebooting the same thing happens.

Windows 10 clean install.

Yes it's the same driver (latest available) on both the disks.

I thought about that but then remembered that I had tried various versions and they all worked, only most of them wouldn't allow the brightness to be adjusted except the original bootcamp driver. I did however manage to get the latest driver to work by entering the enable brightness DWORD in the graphics card section of the registry. So essentially the problem now could be with the startup settings before windows loads that are causing the Nvidia driver to crash. I don't know if it is possible to control the startup settings using clover boot manager in a way that it sorts out this issue!?