Try Power Quest’s “Partiton Magic”.
Create a hidden primary partition on the hard drive. You can load your own personal operating system on the hidden partition.
It comes with “Boot Magic” that lets you select which partition is used to boot, and can be password protected.
When your primary partition is active the other partition that your dad messes with is hidden to the system. The reverse is true for your dads partition.
Bonus is you can resize partitions without redoing the hard drive.