Task manager showing 100% CPU usage at idle.

Hi Audi. I'm Greg, an installation specialist, 10 year Windows MVP, and Volunteer Guardian Moderator here to help you.

1. See common fixes here for 100% CPU use to make sure you've covered them:

http://www.thewindowsclub.com/fix-100-disk-usag...

2. Go over this checklist to make sure the install is set up correctly, optimized for best performance, and any needed repairs get done: http://answers.microsoft.com/en-us/windows/wiki...

Start with Step 4 to turn off Startup freeloaders which can conflict and cause issues, then Step 7 to check for infection the most thorough way, then step 10 to check for damaged System Files, and also Step 16 to test a new Local Admin account. Then continue with the other steps to go over your install most thoroughly.

3. Update your drivers and BIOS/UEFI firmware from PC or motherboard maker's Support Downloads web page, using the full model number, Serial Number or Dell Service Tag on sticker. Compare the latest drivers available for download with the ones presently installed in Device Manager reached by right clicking the Start Menu. Make sure you have the latest BIOS or UEFI firmware, Chipset, Display (Video), Sound, USB3, Bluetooth, Network and all other drivers, or else download and install the latest now.

Updating drivers from the Device Manager or Windows Update is rarely enough. The most basic thing we need to learn to maintain a PC is keep the latest manufacturer's drivers installed from their Support Downloads web page.

4. If nothing else helps you can run a Repair Install by installing the Media Creation Tool and choosing from it to Upgrade Now. This reinstalls Windows in about an hour while saving your files, apps and most settings, solves many problems while also bringing it up to the latest version 1903 which you need anyway and by the most stable method. http://windows.microsoft.com/en-us/windows-10/m....
5. Test the RAM overnight to stress it with the best bootable test memtest86, following these steps which test both the sticks and the slots: http://www.sevenforums.com/tutorials/105647-ram...

Run Crucial System Scanner to check the RAM is properly spec'd and configured: https://www.crucial.com/usa/en/systemscanner?cm...

6. Test the hard drive or SSD with diagnostics:

https://www.lifewire.com/free-hard-drive-testin...

https://mashtips.com/ssd-health-test-and-perfor...

HP and Dell have hardware diagnostics triggered by tapping the ESC or F12 keys respectively as you power on.

Best is to use the HDD/SSD manufacturer's own if there is one, otherwise use Seatools bootable long test: http://blog.nowherelan.com/2013/04/04/boot-seat...

Follow this up with a full Disk Check: https://www.groovypost.com/howto/check-hard-dri... if necessary from the Command Prompt at boot: https://www.tenforums.com/tutorials/2294-advanc...

7. Compare the install you did with this gold standard Clean Install of Windows which will stay that way as long as you stick with the tools and methods given, has zero reported problems, and is better than any amount of money could buy: http://answers.microsoft.com/en-us/windows/wiki...

The reason I ask you to do this is because no one of millions who have followed the install in that link have ever come back to report problems that I have seen. It is also a great learning experience that will make you permanently the master of your PC because you will learn what works best and have applied it with your own hands.

I hope this helps. Feel free to ask back any questions and keep me posted. If you will wait to choose if I solved your problem, I will keep working with you until it's resolved.

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