Always keep the original CD for the operating system because it is made to be used as an emergency startup disk for those times when, for whatever reason, your computer won't boot. You can usually at least boot from this disk (if not, you have a more serious problem), and then take a look at your System Folder and perhaps find something obviously wrong, like a Finder missing. You can trash corrupted fonts or files, remove extensions, and even reinstall your System.
To force the Mac to boot from the CD, put the CD in the drive, restart (see next page), and instantly hold down the letter C key and keep it held down. You can tell if the CD is running the machine because its icon will be in the top-right corner instead of your normal hard disk.
Occasionally the computer screen just freezes. The pointer may move around, but you can't click anything and it doesn't respond to the keys. If you can hear your hard disk chugging away, the Mac might have just gotten hung up on something--wait a minute or two, and don't click on a bunch of stuff because then the computer will try to do those tasks as well. If it lasts more than two or three minutes, it's got a problem. Sometimes static electricity can cause this, or a conflict inside your Mac, or sometimes it happens when the computer runs out of memory. You can try pressing Command Period, but it rarely helps.
What is sometimes effective, though, is a force quit. Press Command Option Escape (esc). You will get a dialog box asking if you really want to force the application to quit. Well, you have no choice. Anything you had not saved in that application when you force quit is lost.
If the force quit in your application actually works, it's a good idea to save and quit in all other open applications and then restart. If force quit doesn't work and everything is still frozen, Restart...
Restart after a Crash
Restart: hold down the Command and Control keys, and push the Power key (the big one at the top with the triangle on it). This is safer for your computer than turning the power off and on. Some newer towers don't have a Power key: press the Power button and hold it down to the count of five, then push it again to restart.
Most iMacs cannot restart with the key combination above. To restart an older iMac, straighten a paperclip and poke it into the restart hole, which is inside the side panel; it has a triangle above the tiny hole. Newer iMacs have a little button to push in that same position.
Figure 12 On older iMacs, there is a tiny hole underneath the triangle symbol. Poke the paperclip in the round hole.