Believe it or not, this machine was originally running Vista, which was upgraded to 7. I then did a fresh install of Windows 8 when my old SSD C: drive broke, but managed to dual boot - not that I ever use Windows 7...
I could load Windows 7, but the 'mentions' of Windows 8 are erroneous as I'd upgraded to 10. So I wasn't able to boot into 8, but was able to use the Change Defaults... - Choose Other Options - Troubleshoot - Start-up Settings method from the boot menu to get into Windows 10 Safe Mode. That's how I knew Windows 10 was still there, somehow... ;-)
I was eventually able to uncheck Use Original Boot Menu, but the damage had been done...
...Anyway, I was able to get Windows 10 working again. I downloaded EasyBCD (
EasyBCD - NeoSmart Technologies) and Dual-boot Repair 10 (
Dual-boot Repair Windows 10). The latter saw that there was a Windows 10 installation that wasn't a boot option, but Automatic Repair didn't work. So I used EasyBCD to create a new (Windows 7/8!) entry, and made sure that everything was pointing to the C: and not D: drives. I then used Dual-boot Repair MBR & Boot Records, and to Repair BCD. I'm not absolutely sure I needed all of these steps, but, one way or another, it worked!
I'm now, much to my great relief, back in Windows 10, and shall be sending suitable contributions to the makers of those two programs. ;-) I'm also very grateful for everyone's help here. It was a waste of a day, but I'm now back on track... (If I'd done what the MS chap had suggested, though... (shivers))
I'm now returning to my original problem, and have successfully performed a clean boot. The PC was still working when I came down this morning, so I'm now convinced - as per your original thought - that it's something loading (either via Services, or via Startup) that doesn't agree with Windows 10. I'm going through these items in groups of about five until I can force a 'freeze', and hope (eventually) to be able to isolate the culprit.
I've also uninstalled and am in the process of reinstalling my AV and Firewall software.
I'll let you know when I have a solution. Thanks again!