Setting up folder restriction's on a computer?

This has to do with parental control, but not the type for the internet.
On Window's 7

I have built a computer for my children to use. I don't want them tampering with any file's or downloading anything without permission though.
So naturally, I have come up with only allowing them to have access to the My Pictures, My Video's, My Music, and My Document's folder.
Now I assume the best way to do this is just have a password protection for every other folder. I would like to just have one user account on the computer. So, if possible, avoid having a parental and child account.

Is this possible? And if so, how can I do it?
If it is not, then does anyone particularly know a link on how to do this?

To reiterate, my concern isn't on internet restriction (at least, not yet); but on download restriction. My oldest child has already loaded up an old computer with so much adware etcetera, rendering it useless.
So it's to control what they want to download through me.

Looking for the simplest solution

(Making it so only I know the password, and so only I can log-in and download/install what's needed.)