After Windows 10 update PC is very slow with disk always 100%

[prrd12]

My son's PC upgraded to W10 and was running fine for 1 month. After an update 2 weeks ago, it slowed to a crawl taking 30-60 min to finish startup and 10 min to launch apps. Rolling back to Windows 8 didn't help. Malwarebytes didn't find anything but Chrome/Firefox stopped loading so I am limited in doing further scans. The only thing that works in real time is the mouse cursor. Based on the taskmanager, the CPU use is low but the C: disk is always at 100%. I'm not savvy in this and my son is out of town. I don't want to wipe all his stuff with a reset. Any ideas on where to start? Thanks.
Specs
Intel I7-4770 3.4Ghz
8 GB DDR3
Nvidia Geforce GTX 760 2gb
1TB SATA

 

[Slashgeek]

It appears that something terribly wrong has gone with you underlying Windows OS. Download the Windows media creation tool here:

https://www.microsoft.com/en-us/software-download/windows10

Use it to create a USB or DVD Windows installer. Use your new installation media to perform a clean "Custom" installation over your old OS. This will not result in any file loss.

 

[prrd12]

Thanks, so much. The PC is working again.
I made the iso DVD and "custom" installed. I couldn't select a partition initially but it worked when I changed the boot option based on the good advice from Richman.
http://www.tomshardware.com/answers/id-2124541/windows-installed-disk-selected-disk-mbr-partition-table-efi-systems-windows.html

Most of the previous files are in an "old" windows folder or the E drive. I am not sure how to get the programs working/installed from there but I will leave that to my son.