You might be very careful and do almost everything you can think of to make certain no other person has access to your computer, but some person might be able to obtain access. It could be someone who is on a network you use, a co-worker who is at your PC when you are away from it, or a hacker.
One good way to solve the problem is to encrypt the data you don't want to be seen by other people. A bad thing is that many of the encryption programs are a little costly and some of them are a little difficult to use.
There is one that is easy to use and is also free. When it is installed it creates a new encrypted volume on your PC. You can move files into that volume and they will be encrypted on the fly, or you can create files inside that volume. You don't need a password to work with these files the same as you would any other files.
You highlight any files you want hidden from others and click the Unload button. The files will disappear. To see them, you click the Load button and type in your password. People who don't know your password won't be able to see those files.
You can also use this program with small USB flash drives. If you encrypt the whole drive no one can see the files if you lose it.
This particular encryption program is from Cypherix and it is called Cryptainer LE. Here is a link to it if you want to get more information or download it and give it a try
Cypherix Downloads - Cryptainer LE Download Center