Windows 10 has a bunch of new apps, including a new Photos app. I don't mind the Photos app -- it certainly has some nifty features, such as the ability to edit and enhance your images with a couple of clicks -- but I love the old Windows Photo Viewer. And I miss it. A lot.
I miss it so much that I made it my default photo app. And here's how you can do the same thing.
If you upgraded from Windows 7, 8 or 8.1
It's easy to get the trusty old Windows Photo Viewer back -- simply open up Settings and go to System > Default apps. Under "Photo viewer," you should see your current default photo viewer (probably the new Photos app). Click this to see a list of options for a new default photo viewer. Assuming you upgraded to Windows 10 from a previous version of Windows, you should see Windows Photo Viewer as an option.
Choose Windows Photo Viewer and exit the Settings menu, and you're done -- photos will now open up in Windows Photo Viewer.
If you installed Windows 10 from scratch
This is trickier, because while Windows Photo Viewer is still technically available, it's not easy to find. IT does involve editing the Windows Registry, which you should not attempt unless you know what you're doing -- editing the Registry can be dangerous if you make a mistake.
Here's how to do it:
1. Copy the code from this post by Edwin over at TenForums into Notepad and save it as a REG file (call it whatever you like, maybe photoviewer.reg).
2. Double-click on your new REG file to merge it with your Windows Registry. You will need to click through the User Account Control and a few other windows to allow the file to make changes to the Registry.
3. You should now be able to see the Windows Photo Viewer and set it as the default program for various image files. To do this, open the Control Panel and go to Default Programs > Set Default Programs. Find Windows Photo Viewer in the list of programs, click it, and choose Set this program as default. This will set the Windows Photo Viewer as the default program for all file types it can open by default.