Hello Norm,
Thanks for your reply.
I have not found any reference to "ruby.exe" in my startup programs, either in the registry or in the programmed tasks. In addition, there's only one user profile defined on this computer, and it's (of course) an administrator. I also checked my user's permissions on the AppData\Local\Temp folder, and my user ID have full control – as I expected.
Finally, this problem only occurs occasionally, but when it does, it's always on the first logon after I power up my computer in the morning, and most often on the first power-up after a Windows Update or a new build has been installed (I am a Windows Insider, slow ring). I was rather afraid it was some malware that hadn't been caught by my regular scans with MBAM, SAS and EEK.