Can you tell me how to do a differntial image using Windows 10 and with the what free software?

This may need a lot more detail from you. I've had folk ask such to find out why Windows is not booting but given the boot area is not something a normal tool will tackle, this is just one of a few areas that file comparisons won't tell you a thing.

So, WinMerge can compare two drives but it also means that the two drives under inspection are NOT being used to boot Windows.

Finally a big area of fail is the registry. To compare that you would have to export both then try WinMerge to see where the changes are. My bet is you will understand why they say this is like taking a sip from a firehose.

-> With that in mind, can you tell more about why you need to do this?

I use Macrium Reflect (free home edition) and I have it set to automatically do a full image of C: at the start of the month and a differential every day.

You can download the Daemon Tools Ultra. The software has all the necessary tools. I think that a three-day free period is enough for you.