cannot log in to windows

Is there any other user name you can use to log in?
Restart (to slow it down) and press F8 repeatedly. See if you can get to Safe Mode and then log in.

Possibly, the drive letter for c: has changed. Boot windows setup, mount the registry off the offline system and delete the key "mounteddevices", then reboot.

I guess you will need to help me with the procedures for mounting the registry.

Did you get into Safe Mode?

Tapping f8 doesn't get me to safemode.  This is windows 10.  I use the windows setup and tried to use the command :D:\windows\system32\msconfig.exe" to change the setting to start in safe mode but it says I need to have elevated command to do this and I do not know how to do that in the command mode

Right click on the command above containing MSCONFIG and see then if you can Run as Administrator. Then you have the elevated mode and can get into Safe Mode that way.

All we want from Safe Mode at this point is: Can you log in?

To get to safe mode boot to the logon screen, keep the shift key pressed, while you click on restart. It will offer additional startup options including safe mode.

I'll look up the instructions for registry mounting, hang on.

Yes, I am trying to get logged in but it won't log in at the welcome screen.  Like I said earlier: When I click on the name it tries to log in but then logs right back out.
I cannot get to safe mode.  I have tried several different ways but nothing works.  I am just going to use the reset and method and see if that helps.  I have spent all afternoon trying to get into the computer and after all of this I could have re-installed the system and had it going by now.

If you have a backup or if during the reinstall mode you can keep the data that may work.

Caution. I have had an ongoing issue with right mouse context on my ThinkPad. Friday morning I said reinstall keeping the data, it did so, going back to my own user ID. It was all working fine last night.

But if you cannot log in (as I did) you may have to just start again if McKnife's suggestion later do not help. Reset (as you suggested) and Keep Nothing .

That means losing your data.

Ok I held the shift key down while selecting restart and I got to this screen that I uploaded but the thing i can do is select restart and then it just starts up in normal mode - right back where I started.
20180804_173356.jpg

Thanks for helping with this issue.  I am getting highly suspicious of a faulty hard drive.  Monday I am going to pop in an SSD and see how it goes.  This drive is fairly old, a blue Western Digital, which I have not had too good of luck with in the past.  I was hoping there would be a quick fix as the customer was wanting her computer this afternoon, but it didn't happen.
Thanks again for your help.

As for my instructions which you didn't wait for before closing:

1 boot setup to the language selection screen, press shift F10 so that a command line appears
2 launch the command regedit on that command line
3 in regedit, select the branch HKEY_LOCAL_MACHINE, click on the file menu and select "load hive" and navigate to the file c:\windows\system32\config\system and you'll be asked for a name, call it "offline"
4 within the offline branch that is now mounted below HKEY_LOCAL_MACHINE, navigate to the branch system and below it, delete "mounteddevices"
5 select the offline key again, go to the file menu and select unload hive.

After unloading, reset your computer and try to logon.

Good luck with the new drive.

Thank you, Mcnife - I haven't done anything since saturday night, so I will try that.

To Mcnife:
I did as you suggested and that didn't work either.  After doing another clean install everything seemed to go great - I shut down and restarted the computer several times and it worked fine.  The next day the customer came to pick up the laptop and it wouldn't even start up.  It claimed the boot records were either corrupt or missing.  I am going to try and sell her a different and newer laptop as this one is rather old.  but I do thank you for your help.  Even though the real issue wasn't fixed I still learned quit a bit.

These symptoms point to defective hardware, most probably hard disk or RAM, indeed.

new updade (and possible final update)  i got to thinking that maybe I should just go back to windows 7 and see what happens.  so that is what I did and everything seems to be working perfectly.  the customer picked up the laptop and so far has had no complaints.  It appears the laptop was too old for windows 10

A number of Windows 7 machines are 5 or more years old and not engineering to run Windows 10. So that is probably the issue here.