How do you do a custom encrypt on the files in your system folder?

[Salty Mike]

I'd like to ask if there is software that encrypts the files based on a custom key you can choose and then you can only decrypt using that key?
Also, how is the whole custom encryption done in general?
I need to protect my work in the client (it is not ready just yet), but I've just recently started working on the client side of things, so everything is quite new to me.