Try checking for a process called "superfetch" when you start your PC next time. It's one of the main things that causes 100% disk usage and since you said it begins with you turning your PC on I'm most assured it's the main cause of the issue. The programs and other processes you are running in the background are written in your RAM memory (hence RAM caches), superfetch is basically making your hard drive to fetch those programs from memory to load quicker than usual. Don't be alarmed, it's not harmful at all and you can disable it if you want.
- Roohan Nawaz likes this