You could always set the BIOS to prompt for a password at bootup. Set the password, and then look for an option (Probably in BIOS Setup or Advanced Setup) that says "check for password at bootup/setup" and change it to "bootup." Every time the PC powers on, it'll do the POST and ask for a password before booting to an operating system.
Of course, if the person can open a PC case and find his way around the motherboard, he can always just use the reset CMOS jumper.
What version of Windows are you running? If WinXP or Win2k, securing the system is much easier than with Win9x.