What’s the best way to handle a laptop that might have malware?

I'm trying to help a friend with their Lenovo ThinkPad. It has an Intel i7 and 8+16GB of RAM, but it's been crashing and showing the blue screen of death multiple times a day. Lately, it crashes almost immediately after the login screen. We noticed that the CPU usage spikes to about 70-80% on startup, mainly due to the 'Windows Anti-Malware' program, leading me to believe the laptop might be infected with malware, but I'm not sure where it came from. I attempted to boot into safe mode, but instead of entering the command screen, it just restarts normally. Given that the laptop can't stay on for more than 20 seconds, I'm wondering if there's any way to fix it. I did notice that it's more stable when plugged in, but the CPU is still running high with anti-malware using most of the resources.