How To Identify Cause of 100% Disk Usage?

I have a win10 machine that keeps slowing down drastically and when I check with Task Manager I see there's 100% system disk usage.

And I can't figure out what is causing it.

And I can't understand how it could be that important that is would slow the machine.

100% usage simply means that the disk is being accessed all the time doesn't it? That's all. It gets no rest.

But CPU usage is fairly minimal. 97% idle time. Disk transfer rate is low.

So I figure it must be something that can't operate unless it can access the disk and it simply can't access the disk.

But all I am doing is browsing web pages. Reading them. Forum posts usually. And I might write a post. Not disk intensive activities.

I look for I/O and page faults and find Firefox and Avira to be most heavy.

Is there any way to identify which processes and therefore which applications are being delayed?

I've got two machines with win10 and each set up pretty much the same and each used in about the same way.

This one here now never gets this problem. That other one I've had endless problems with. It's an Asus H170 pro gamer. This one is an Asus H61-MK. Both Asus, see? Even that much similar.

aaarggh... my god... right there before my eyes on the front page ! I've been giving it a quick glance and tearing past it looking for some kind of specialised in-depth page that will tell me....

blinding myself with science...

Brilliant. Well it seems to be Avast (not Avira i've got, but Avast). Hogs the limelight. Then Service Host Cryptographic and Diagnostic and Prefetch and Win Problem Reporting.....

Avast right at the top of the list. I think I'll throw it out.

This machine that works so beautifully and shows none of this congestion and commotion - it is showing zero disk use right now - doesn't have avast.

So a big thanks for that.

Any thoughts on the diagnostic suite?