You can reinstall your operating system from the DVD that came with your Mac.
Start from the DVD by holding down the "C" key on startup.
Connect you Mac in FireWire Target Disk Mode with another Mac if regular reinstall doesn't work.
Make sure you connect a PowerPC Mac to another PowerPC Mac, or Intel Mac to Intel Mac.
What to Do When Your Mac Is Really Screwed, Or How To Reinstall Your Mac OS XCategories: Getting Help
Ai-yi-yi! Your Mac keeps crashing and won't operate. This isn't supposed to happen, right? Maybe it's not even starting up all the way. What do you do then?
If you can, go through My First Mac's 6 steps in Oh-No, My Mac Won’t Start. Now What!? But what if your Mac is still dead and those steps don't apply? It's time to go nuclear, and by that I mean it's time to reinstall the operating system on your Mac.
This is when you need to locate that DVD that came with your Mac so you can do a reinstall. If your Mac will run and recognize your install DVD, insert it and run the installer. It will restart your Mac using the DVD as the main disk and let you install wipe your main hard drive. If your Mac isn't starting from your main hard drive, it still might start from the DVD. Insert the disk and hit the power button while holding down the "C" key. That tells your Mac to start from whatever disk is in the optical drive.
At this point you can follow along from the installer instructions. Don't forget to hit the Options button where you then can select “Archive and Install,” and then select “Preserve Users and Network Settings.” This will save your old files and some settings.
Sometimes your hard drive is hosed and you'll need to do an Erase and Install. When you select this option, the drive will be erased, reformatted and the install will be brand new. That means that you will lose ALL the data from your hard drive. This is when you are really happy you've been backing up your data or really pissed that you haven't been. If you've been using Time Machine, you can easily get files back from there.
Really, Really Screwed
OK, what do you do when your Mac won't even get far enough along to start up from DVD? It's time for start your drive in FireWire Target Disk mode. What happens in this scenario is that you restart your Mac holding down the "T" key, enabling Target Disk Mode. When you connect your Mac to another Mac with a FireWire cable, it responds like an external FireWire drive
At that point, you launch the installer from the second Mac and select the "disk" that is the first Mac as the destination for the install. It's the same procedure as if you were to install the operating system on an external FireWire as a backup. By the way, that is a handy thing to do so you always have another option for starting up your Mac. It's also a good way to keep older versions of Mac OS X around in case you need them.
Really, Really, Really Screwed
This really happened to me. I went through the above steps with my 5 year old PowerBook I'm writing this on a short while ago. I had to do Target Disk Mode because my optical drive is out on the PowerBook. When the install came to an end, it told me it couldn't finish the install on my PowerBook hooked up to my Mac Pro. Or from from my MacBook. After a lot of searching on the web, I found out that an Intel based Mac can't run an install on a PowerPC based Mac. I figure that the install disk has 2 versions to boot from, and if it gets booted from an Intel Mac, it can't install a PowerPC Mac version. I don't know why, but that's the way it is.
At that point I realized I had to track down ANOTHER working PowerPC Mac, so I went to visit my Friend's G5 PowerMac. Once I hooked up the PowerBook in Target Disk Mode, it all went according to plan. And of course I ran Software update a few times, until the update that originally wrecked machine did it again. And then I started over. Again.
So before you kick your Mac to the curb or throw it through a window, don't forget that a reinstall will cure most any software problem. It won't fix your hardware, but it should get your Mac back to square one.
Do you have any war stories to share? Let us know in the Comments section below!