Hey there!
Last Moon doesn't contain any viruses and the download files haven't been touched in years so I don't think ya need to worry in that regard!
These errors usually have to do with core data from your pc itself being somehow modified so I highly suggest trying to run a scan (SFC) if you haven't already.
Do you recall if you had a Windows update around that time? Unfortunately I've found quite a few people have had this problem with Windows 10 updates lately. You can also try to boot your PC on safe mode to help make it easier to navigate when trying to solve the issue.
I recommend looking over the windows websites for more solutions since that's a bit out of my reach!