Option A) Use a linux Live CD / USB. Some are specifically geared towards privacy and security, like Tails. (the better option IMO)
Option B) Simply use any hardened Linux distro, disable SSH server if it's enabled, and keep it updated as well as using a good password. Any of them will be superior to Windows 10 with an antivirus IMO.
Option C) Clean install Windows 10 without internet connection, install some form of reputable internet security like ESET or Bitdefender ( I don't like Kaspersky, it intercepts HTTPS traffic ), install W10Privacy and tick all boxes you wish, apply, reboot, activate Bitlocker and Secure Boot, THEN install a browser and connect to internet, and keep it powered off or with internet disabled as much as possible, and keep it updated as well as using a good password.