System Internals at Microsoft has a large toolkit of pretty capable tools.
Process Explorer is a much more capable version of task manager.
Rootkit revealer can help uncover more sophisticated malware that isn’t caught by defender.
The sysinternal suite link downloads a single zip file with most of the sysinternals tools.
These tools can give visibility into the internals of windows and diagnose problems. Doing diagnosis successfully will require a good bit of learning curve and digging but the more you learn the more you can do and much of that knowledge comes in handy when doing system level coding as well.