Hold down the Alt (Option)-Apple-P-R key combination while rebooting the Mac. Keeping this combination held down will cause the Mac to chime repeatedly until you let go. Wait until you've heard three chimes (which should take about fifteen seconds or so) and let go of the keys, allowing the Mac to continue booting normally.
NOTE: PRAM (Parameter RAM) is a 256 byte section of memory that is constantly maintained by a Lithium battery within the Mac. This memory is used to store some basic information such as date & time configuration, AppleTalk port settings and memory settings. The most common signal that the PRAM needs to be reset is when you notice that the Mac forgets which devices are attached to which ports, or even more obvious, when the date & time resets to 1904, 1936 or 1970. Zapping the PRAM actually only clears the top 64 bytes of the total 256 bytes present. The lower 192 bytes are deliberately protected, though applications such as TechTool can be used to completely clear the PRAM if necessary. Unlike rebuilding the desktop database, zapping the PRAM should only be done when it is needed, not as a general housekeeping measure. Zapping the PRAM for no good reason may cause more problems than it resolves.
NOTE: On some occasions, the PRAM battery will be worn down to the point where it is unusable. This is relatively common on old Performa series. In these circumstances, no amount of zapping is going to sort things out. Apple retailers should be able to provide you with a new PRAM battery for less than fifteen pounds.
