Windows 10 is locked up at log in

I upgraded my parents (they are in their 70s and fixing their computers has become a second full time job for me) to Windows 10. Probably a big mistake but I was hoping it would solve some issues I was constantly having to fix. They have Verizon internet and it is extremely slow so when an update was released, I am afraid it probably didn't complete so now I have a issue I cannot fix. I've brought their computer to my house hoping a faster internet would help but so far all I can do is log in, that's it. The task manager isn't responding, I cannot get to the settings or preferences. I've tried rebooting with a Windows 7 disk that my son had from his computer(their system when purchased from Best Buy did not come with a disk so, there's that), I've also tried rebooting while hitting the F11 key (HP) to try to set back to factory settings but no luck.. it will go to the log in screen and then do nothing. Last night thanks to my keyboard, I was able utilize shortcuts to access Word and Excel, so I do know some things are functioning. Also McAfee is working and iTunes did try to update. However, that's kind of useless if you can't get anything else to work. Does anyone know what I can do to fix this? We don't care about the files saved, they were backed up on an external hard drive. Any help is greatly appreciated. Thanks!

You may have to change the BIOS to boot from disc first. I believe that's F10 on the HP's I've worked on recently.

If nothing needs to be saved, you may consider a clean install right to Windows 10, but you'd have to use a functioning computer to download the Media Creation Tool. That will allow you to make a disc or a bootable USB drive to install Windos 10 clean.

Does their computer have a visible sticker showing a license key?

I'll have to check tonight, at my real full time job right now.. I do have a couple PC's I can download a clean copy at home (duh! should've thought of that last night, but between it and them, I'm a tad flighty). I'll update the thread later tonight.. thanks for the response..I really appreciate it.