Deep system malware detection

This might get taken down put I will attempt to ask here. As an intermediate level PC nerd with no experience in writing programs, can you guys explain how rootkits and undetectable malware work? I strongly believe my PC may be affected with said threats due to high CPU usage (90% with no programs running), battery drain, PowerShell popping up then disappearing, computer not sleeping when the lid is closed (it will be dead by morning) and every single account …

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