"Hacker proof" is a bold statement.
No OS is 100% "Hacker proof".
However, Linux's security system, by default, is very good.
Both GRUB and Lilo (linux bootloaders) can have a password which prevents access and modifications.
This means no one will be able to boot Linux without a password.
If you want more security, you would make your HDD the first boot device and set a BIOS password.
And even then, the BIOS password can be cleared by switching a jumper on the motherboard.
In the end, if someone is actually in the same room as the computer, they may as well just steal the entire PC (or at least the hard drive).
If you want to protect your data, you have to encrypt it.
If you have access to a hard drive and the data on it is not encrypted,
then no matter which OS you use, the data can be accessed.