How about using a BIOS password?
When the system is first turned on, you'll get an option to press F1, or Delete,
or some other key to "Enter Setup" or similar phrase. Have a look around at the
various screens for password or security settings. There will probably be two
options; System and Setup passwords. "System" will ask for a password every
time the computer is turned on. You may have to "set" the password in one place,
and "enable" it in another, depending on the machine. If you set a password,
MAKE SURE you remember what it is, and whether you used upper or lower-case
letters/numbers, or you may lock yourself out!  :(

If you get confused, or change something you didn't mean to,  just keep hitting
Esc until it asks if you want to Exit Setup WITHOUT saving changes.