Using the system we have now. That is, a login password. Without it they can't get on your PC and I'll ignore the workaround where I boot Linux to get at the files since your solution breaks a bit too easily.
So why would I need this if we use what Windows supplies and we are sticking to just Windows?
I know Bitlocker and used it my PC and portable drives, But this time, I am looking for a lightweight solution to protect my private folders. When someone tries to access the folder, a password prompt appears.