Since a fresh install or rather factory reset didn't help, it's likely to be hardware. Could be as simple as a memory stick came unplugged to heatsinks that fell off or cracked compound.
I'll take your word you have a factory install of Windows but without make, model and a report (link to follow) all should work fine if it's software.
One evening my computer worked fine, the next day it was unbelievably slow. I tried lots of things antivirus system restore both 2 and 4 days back but all no good. I reinstalled windows. I have windows on my C drive which is 120Gb SSD and my data on a separate 1Tb drive. After reinstalling windows things are no better and additional access to some folder on my E drive isn't allowed. I have reinstalled windows several times but never had that problem.
I don't know where to go from here. Have tested memory and hard drives all OK.