Windows 10 .iso association disappearing

Thanks for posting back your specs so quickly! We appreciate that.

Yes, hard to say without having your PC on my workbench. But several things can cause this to occur.
Here's a short list:
1.) Virus/malware attack (did you scan your PC with your AV program?)
2.) Windows corruption from virus/malware or from the process of removing a virus/malware from your hard drive
3.) General Windows system file corruption due to stubs left in Registry after program removal/uninstall
4.) Failed Hardware: usually Hard Drive, or RAM sticks
5.) Motherboard failure

System Restore only goes back 5 months; so if your problem has been occurring longer than that (and you're just getting around to fixing it now), you are outta luck!

If you are not using Backup Image programs (not the Windows Backup! ugghhh!), such as Macrium, Acronis, or EASEus ToDo; you could easily restore your hard drive to working condition by going back to a point in time things were working prior to 5 months; such as 6 mos., 1 yr. or longer ago. You would lose some data and programs but you would solve your problem within a few hours. If you are not using this method, you should look into it if and when you solve this immediate problem!

What, no Image Backup?? Not to worry; you need to start here.

STEP #1: Before attempting any further repairs we suggest, FIRST BACKUP ALL OF YOUR PERSONAL DATA TO EXTERNAL MEDIA! THIS WOULD INCLUDE YOUR LIBRARY FOLDERS FOR DOCUMENTS, PHOTOS, MUSIC, VIDEOS, AND ANY SAVED E-MAILS OR ATTACHMENTS TO AVOID IRRETRIEVABLE DATA LOSS!!!

STEP #2: Use whatever your built-in AV program is (Norton, Avast, AVG, McAfee, Windows Defender) and scan/remove all found viruses. Next go to malwarebytes.org and download the free MALWAREBYTES antispyware virus program. Scan/remove all found spyware viruses. Reboot and retest for the problem. If it works, you had a virus or a spyware virus, you've removed it and repaired the damage, and are good to go!

STEP #3: You can use Windows reset or Windows Reinstallation; both rather drastic steps (no shortcuts if you have #3 or #4, sorry); but first you MUST TEST YOUR HARDWARE (RAM sticks and Hard Drive). Even if you think all your hardware is Ok and you bought all the parts new; doesn't matter, they are at least 4 years old and could be as old as 6 years old. Hard Drives for instance in desktop PCs are only designed to last 3 years! This means it's quite likely 1 or both of your hdd's are failing or failed. And that's another thing; you SHOULD NEVER TEST YOUR PC WITH ANY MORE THAN 1 HARD DRIVE CONNECTED TO THE MOTHERBOARD (SATA, eSATA, USB, etc.)!! That's a no-no! You should disconnect your 2nd drive, either the Hitachi HDD or the Samsung SSD from the Mobo when testing; whichever is your bootdrive (C: drive) where your Windows files live, should be the only drive used when troubleshooting this problem.
To aid you in testing your hardware, I wrote a step-by-step Troubleshooting Guide which will help you accomplish this and is available here: Windows 10 - Unclickable Task Bar

STEP #4: After your hardware testing has passed (or failed but you replaced all failed components to get a Pass), you can click on the 2nd half of my Troubleshooting Guide (SOUND PROBLEMS) link in the above post. This will take you to the steps you need to Reset your PC using W10 bootable media (if you don't have it, you can google how to make it yourself), and try that. If that fails, you are probably looking at a complete W10 Reinstallation from Factory Recovery Media! The Guide tells you how to do this. Since you built your own PC, you may have done this before. The big problem for self-PC builders is that your W10 media was either downloaded via W8x to W10 upgrade and you don't have the file, or you have Microsoft legit W10 retail media, and reinstallation will cause you to lose all your INSTALLED PROGRAMS AND DATA!! Your data will be saved to external media if you followed my Caution at the beginning of this post. Hence the comment about making W10 Image Backup Files using the free utilities I mentioned above just prior to STEP #1.

I realize these steps are drastic, but if you brought your PC to me, this is how I would go about repairing it, as would most licensed Computer Techs. If you are having trouble wrapping your brain around this whole thing, you might consider taking it to your local Computer Tech and paying them to do the above process. Keep in mind my Guide is 6-8 pages long, so it's a good idea to print out the entire Guide and have hardcopy in front of you while you are working on the broken PC. Also, the troubleshooting process can take 2-3 days to 1 week or more depending in your experience level, so have Patience!

The above steps will definitely fix the problem you are having.

Oh, and by the way, did you attempt to read the problem ISO files on a different computer you have or that you can borrow such as a friend or neighbor's computer? Same result or did it work on their computer? If it didn't work on the borrowed computer, those ISO files could all be corrupted, or infected with a bad virus, so make sure you scan them if your reset your PC or reinstall W10 prior to putting them back there! Otherwise, the problem will reoccur all over again.(and hopefully, none of those ISO files were downloaded from P2P sites such as uTorrent or Pirate Bay or the like-those files are ALL LOADED WITH SPYWARE VIRUSES THESE DAYS!!

Let us know how it goes, or if you have any questions along the way. We are here 24x7x365.

<<<<BIGBEARJEDI>>>>