Laptop fan noise

Gillor

Probably best to start with Process Explorer (Process Monitor can initially seem to be a little complicated).

Take a look here for a guide as to how to use Process Explorer.

Process Explorer can be considered a supercharged version of Task Manager... both are Windows tools.

grumpycrab

Don't know if you solved your startup mystery but Glary Utilities has a "startup manager" that shows which processes run at startup (and how long).
http://www.glarysoft.com/glary-utilities/downloadupdate.php

(I used to have a really useful German Startup "Optimizer" which has magically uninstalled itself...)

kwikbreaks

It's more than a "startup mystery" - the OP says the machine is constantly at 100%.

Process explorer looks to be the best bet to me now that malware/pups appear to be eliminated by malwarebytes.

bluebell321

Will try that and report back