Question 100% disk usage slowing down PC/laptop

[trashcan94]

Same issue on both my laptop and PC. Noticed they were slowing down a bit so I checked task manager which said 100% disk usage, nothing in particular seemed to be using much of it though.
I googled solutions but everything was saying "it could be a lot of things so try these 800 different methods and maybe that'll fix it" seemed like a pain so I put it off. I can't remember exactly how long ago but probably 1or 2 months.

Now my pc won't start. I turn it on and it displays the motherboard logo and then goes to a black screen with a flashing white underscore.
PC specs: windows 8.1
cpu: amd fx6300
Gpu: sapphire dual-x r9 280 3gb gddr5
Mb: gigabyte ga-970a-ud3p
8 GB ddr3 1866 240-pin Udimm ram
Hdd: western digital blue 1TB 7200rpm
Psu: cx600m corsair
Also: I use an Asus usb-ac55 for wifi and right before i shut my pc down for the last time(before it broke)it stopped working. The light on the stick wasn't on and I heard the pc make the "usb has been unplugged" sound and the wifi wasn't working anymore. So i unplugged it and plugged it into a different USB slot which worked after I did 'windows troubleshooting' dont know if that's important.
I also bought/started using some logitech g230 headphones not too long ago and have been using them. Don't know if this is important either.
For my laptop I just went and did a 'windows reset' which is just re-installing windows OS (back to factory settings). I had 8.1 but the reset took me back to win8. Obv now I have a million updates to install, and task manager is still saying 100%disk usage. Currently installing updates.

Thanks for reading
any help u can provide is greatly appreciated.

 

[R_1]

this is a PSU not a HDD. (Hard Disk Drive)
what HDD do you have? what capacity? how much free space?

 

[trashcan94]

this is a PSU not a HDD. (Hard Disk Drive)
what HDD do you have? what capacity? how much free space?

Whoops my bad. Western digital blue 1TB 7200rpm
I don't remember exactly how much free space but it was a ton. Probably 700-800Gb free space

 

[trashcan94]

New info: I originally installed windows 8 from a flash drive. I went and got that flash drive, plugged it in, and booted from it to see if I could just do a clean install of windows.

Didn't work. It showed the windows logo for a few seconds like it's supposed to, however after those few seconds it seems like the computer restarted. I saw the motherboard logo again and it went back to the black screen.---no longer true
I tried booting from that flash drive a few more times and it finally worked. I am now able to access the "repair window" option that is usually available in the installation dvd.
I am currently doing a system restore to right before a recent windows update.