Windows Defender problems

This started when I saw an unexplained debug file on my desktop. Deleted it, ran Malwarebytes and a full Windows Defender scan (which seemed to work) but 2 odd things about Windows Defender came up:

Defender used 100% CPU after a few minutes and stayed there until the scan completed
Offline scan wouldn't work at all

I'm on 1903 (build 18362.387)

Any ideas ?

BTW, downloaded Rkill and ran it. The log saved to desktop was completely blank although the message said it ran successfully.

Here are the directions for a Windows Defender offline scan.

Windows Defender Offline Scan in Windows 10

I also use Malwarebytes for testing suspicious behaviour. Malwarebytes Cybersecurity for Windows, Mac, Android & iOS | Malwarebytes

None of the methods mentioned in the tut work. I noticed mention is made of downloading and running it from a USB. Might try that.

Well, things got even worse yesterday when I had trouble restoring from Macrium from the laptop boot menu (no USB either, my bad, learned a lesson). Anyway, reinstalled Windows so I''m starting fresh and have a couple of images from earlier this month that I can restore too if I want. I may just stay with the reinstall though and set it up to my liking as I like tinkering with my machine. The only thing I did recently that comes to mind is I downloaded a free trial of Adobe Elements from the Adobe site the night before last. Can't say if that was the problem or not but at least all is good now.

Appreciate everybody's help, problem solved.