You can use the built-in encryption feature in Windows (excluding Home edition). Create a new folder and move all the files you'd like to encrypt into that folder.
Right-click that folder and go to Properties. Next, in the General tab, click Advanced and select Encrypt contents to secure data > Ok > Apply changes to this folder, subfolders and files.
To avoid losing access to the encrypted folder, create a backup of your encryption certificate and key as recommended. Click the notification and follow the instructions.