Help looking for security threats and unauthorized access

I need some advice on checking my mom's computer for security threats and unauthorized access. As much as I have advised her what not to do, she took a call from a scammer. They made her believe they were a subcontractor from Microsoft. They logged on to her computer. She can't tell me exactly what they did. I fear they installed something. I scanned it for a virus. I plan to look over the firewall activity. What should I look for? Should I reset the firewall to default? She was having problems sending out emails at the time it happened. Verizon reset her password. Would changing the Windows login password help? I fear she gave her ip address and they can now log in anytime.

Wipe the drive with Killdisk. Install Windows. Anything else is advice with not a shred of guarantee of security or stability. Change passwords to Windows and every website she visits.

That scam has been going on for a while. They didn't do it just to have fun and say they can, then leave the system clean.