If an account has a password then other users should not be able to view the contents of that account if their account is just a standard user surely.
Otherwise just set permissions to those document folders to your user account only from the parent folder properties dialogue.
Depends how locked down the PC is. If either the boot device can be changed, or the case can be opened, then the files on the local disk are /not/ secure.
Booting Linux will let you view any files that aren't encrypted, since only Windows cares about permissions.
If you can take the case apart then you can reset the BIOS, or simply remove the hard disk.
Extreme, I know, but if you are serious about security, you use encryption.