Change the "boot order" of the computer.
Run the BIOS setup program and put the floppy drive and ROM drive first in the boot order. Do this by pushing <ESC> <DEL> <F1> or <F2> when you power-on the computer. The correct button varies between computers. Watch for a prompt when you power-on the computer or consult the motherboard documentation.
Some computers will ask for a password before running the BIOS setup program. You can usually defeat this check by opening the case and removing the "CMOS cell" from the motherboard, which looks like a large round watch battery.