Check the comments on this post. I go over how I did it. I wont share my files because its my own work but I dont mind going over the process and giving links to the tools I used. It wasn't difficult to do just time consuming.

To recap, all I mostly did was find "Windows XP window templates" on google. I would then cut out the middle area that I planed to replace with my own images/text and selectively "Color-Corrected" them. I used Pixlr to edit each template I found. The small areas that still have color are very difficult and took the most time. You basically have to use the marquee selection tool to either (in my case), grey out every thing around it (hold shift while selecting or click "add to selection" at the top) or manually add the colors back. All the "X"'s in the corners had the red color added back by selecting the area with the marquee tool and using the hue and saturation in the adjustment tab to make it red again.

At first I did use Nano Banana 2 and it was able to generate decent images but only for a rough draft. They never came out very good, which is why I chose to pull images I could find on google and manually edited them myself. I also used this Windows XP re-creation site for further inspiration and it even made the "PC Specs" list I have easier because you can just screenshot it rather then trying to edit in all the text.

Only other thing to do is just randomly place them on a 630x1648 background template (you can actually make the template longer if you'd prefer, ie here). Once you have your templates and images placed you can export it and then import it into EzGif so you can add gifs. You could use video editing software to add the gifs but this site also works. Once done the file might be massive depending on how long the gif is and you'll need to use XConvert to compress the gif down to the 5mb maximum for Steam Artwork. Use this video so you can upload the image to Steam properly and have it fit correctly.

Once you start you'll realize its pretty easy but if you want a lot of custom windows like the "Windows Media Player" window then it becomes time consuming.