I have a desktop PC running Windows 7, and no other OS, and it was working fine. And since I was going to go down to a friend's house to fix his non-booting Windows 10 PC, I copied Windows 10 from disc to a USB stick (via Rufus, a bootable USB stick creator program), and to test the Windows 10 USB stick was working, I booted up my Windows 7 desktop via the USB stick, got to the Windows 10 menu, and turned off the Windows 7 desktop. I did this because I thought that the Windows 10 installation USB wouldn't actually change anything on the PC until I told it to (which I wasn't going to do, of course, I just wanted to test that the USB stick booted).

Anyway, the Windows 7 PC still works fine, i.e. online works, no viruses or malware, every program I've tried works, but whenever I boot it up, the Windows 7 PC ALWAYS runs CHKDSK at boot, then it tells me that CHKDSK can't check the C: drive, and tells me it's because of some previously installed software, and that I should use the system restore to go back to before the software was installed. You'd think it would tell me what software is actually causing the problem, just like you'd expect the Windows 10 disk to have NOT altered the contents on the hard drive, given that I did not tell it to do anything at all, but not.

Oh, and all of my restore points have been removed.

Googling the problem shows that it's not exactly uncommon, but mostly it's people who dual boot both Windows 7 and Windows 10, and the working solution there is to apparently turn off fast boot in Windows 10, but since I don't have Windows 10 on this desktop (and really really don't want it), then that's not an option for me.

Any ideas, please?