Hold down the Ctrl-Apple -Power key combination. The Mac should restart immediately without warning.
NOTE: This method applies to the old style keyboard shown below. Most Macs running OS 8.1 or below can be reset in this manner. USB keyboards, such as those found on the iMac G3 and G4 series do not have the same Power button and in general cannot be restarted with the Ctrl-Apple-Power key combination. To force a restart on these Macs, use the small triangular reset button on the machine itself. Early iMacs make use of the pinhole reset, so you'll need a straightened out paper handy.
