[nickjj]

1. Firefox

2. U-block origin

3. Let them know they shouldn't ever download files from an email (even from relatives, because realistically non-technical people aren't sending each other files over email and it's likely one of them has malware that is trying to spread)

4. Disable their mic / webcam (they are not going to use it anyways)

The 4th point isn't so much to prevent malware, but it's to protect their privacy in case they somehow get compromised.

Since I've done all of the above, I haven't had to format a parent's machine due to a virus or malware. They run Windows too.