Standard file security is based on the OS's Filesystem security. A Linux (or even another Windows computer) can, by default, read, write, delete, etc anything.
The first rule of security is that if you have physical access to the machine (ie, if you can take out the hard drive or boot from another drive) then you don't have any security. The security only works with the OS itself.
There are, however, ways to improve that security. For instance, you can use folder encryption which will prevent the files from being readable by any user account other than the user that created them. There is all full-disk encryption, with Bitlocker and a few other tools that prevent the entire drive from being accessed if you do not have the right security key.