Secure Folders is a free application which can protect folders in one of four ways.
You can hide them so they're not visible in Explorer; lock them, which means they're visible but can't be opened; set them to be read-only, so they can be opened but not changed; or make them non-executable, preventing users launching any files they contain.
Optional Explorer integration makes this very easy to carry out. Right-click a folder, select "Protect with Secure Folders", then choose the protection type you prefer, or "No protection" to restore normal operations.
Open the main Secure Folders and you're also able to protect new folders, view everything it's protected so far, change the status of individual folders or turn protection off entirely.
To prevent other users of your PC doing the same thing - or just uninstalling Secure Folders entirely - you can also password-protect the application.
Secure Folders doesn't give the same level of protection as using encrypted containers, but it's convenient, easy to use, and will keep your data safe from all but the most advanced attackers.