My brother-in-law has been in the process of upgrading his pc. It's been running great, much faster than before. He has upgraded everything except his power supply and graphics card.

Last night he started his pc, it took forever to load everything in (desktop icons, wallpaper, Taskbar), then he has major lag opening anything. He also had some apps open partially, then close themselves down. After restarting his pc he was having the same issues. Since he lives right behind me I went over and tried working it out. It runs slightly better now, but is still getting major lag inside of applications and having long load times opening anything at all. Things I've done:

1. Checked for windows updates, and downloaded it.
2. Checked his drivers and updated the ones that needed it.
3. Downloaded and ran CCleaner, cleared the suggested things, then did another sweep of temp files/recent typed urls/etc.
4. Checked his hard drives and made sure there was space. His hdd has over 500 gigs available. His ssd that just has a few games on it (plan on setting it up as the boot drive eventually) has 80 gigs available.
5. Checked startup programs and nothing looks bad.
6. Checked background processes and it seemed high (55) but was actually lower than I get on my pc.
7. Ran anti-virus and nothing came up.
8. Used ryzen master to check his Temps, which were fine.

The only thing that was odd was that in the task manager his disk space would show 100% usage off and on, but there is space on both drives and I can't find anything running to cause it. EDIT Also, when looking at this section of task manager I can't find anything using a ton of space listed. Maybe I'm not looking in the right spot?

We are at work right now, but plan on trying to fix whatever is wrong tonight after we are off work. I'm posting here to get ideas on how to approach fixing it. I'm not super knowledgeable with pcs, we just switched over to pc gaming like 6 months ago, so I don't know of it's something blatant I've missed, but any help would be appreciated. Would it be helpful to wipe the pc and fresh install windows? He said there isn't anything on there he needs, and I already have a flash drive with windows boot on it from when I built my pc. This seems like a drastic step, but it's something I tried getting him to do when we got his ssd, and if it also fixes the issue then its not the worst thing.