Can't Boot Up After Software Update -- Advice Appreciated.

Question

Hi all,
I've experienced what seems to be a relatively common problem -- after installing the latest Tiger software update, my machine can't boot up. If I try to log in, it just freezes on the blue background screen. Holding 'shift' to enter Safe Mode just makes the computer shut down. I've tried the advice from Apple listed here:
http://support.apple.com/kb/TS1417
I just got a variety of messages, all of which say effectively the same thing: 'Macintosh HD cannot be repaired.'
What can I do from here, bearing in mind I don't have an install disk. Is there anything I can do without it?
If I were to borrow a friend's Install Disk (they have Leopard), could I delete everything on my computer and re-install Leopard, and have it work? Or will the problems with my iMac carry over to a new installation?
(I suspected something was wrong the morning before my iMac was able to boot -- I was getting 'page in/page out' errors when I tried to extract .rar files, or move large files to an external HD, so thankfully I took the chance to make back-ups of everything).
Any hints or ideas would be unbelievably appreciated -- I'd hate to have to buy a new iMac (for the expense, not because I don't want a new one).

Answer 1

You can erase everything and start over, but you should Zero the Disk first, but also you need an Install Disc, you can buy Sno/10.6 online, for Leo/10.5 you have to call Apple, who may also supply you with a replacement disk for the original if you have your Serial# handy.
For other sources of Tiger/10.4, See Tom's, (Texas Mac Man), great info on where/how to find/get Tiger...
http://discussions.apple.com/thread.jspa?messageID=9755670&#9755670
You might try this a few times to see if you can fix your Disk...
Tough without a Tiger Disk, but try fsck...
To use fsck, you must run it from the command line. Unlike using your mouse to open an application to do something, you'll need to type a text command at the prompt (#) to tell fsck what to do. The Terminal application (/Applications/Utilities) and single-user mode are two examples of command-line interfaces in which you can type such commands. To use fsck:
1. Start up your computer in single-user mode to reach the command line. Hold CMD+s keys down at bootup.
Note: If necessary, perform a forced restart as described in the Emergency Troubleshooting Handbook that came with your computer. On desktop computers, you can do this by pressing the reset/interrupt button (if there is one) or holding down the power button for several seconds. On portable computers, simultaneously press the Command-Control-power keys. If your portable computer doesn't restart with this method, you may need to reset the Power Manager.
2. At the command-line prompt, type /sbin/fsck -fy
(space between fsck and -fy important)
3. Press Return. fsck will go through five "phases" and then return information about your disk's use and fragmentation. Once it finishes, it'll display this message if no issue is found:
** The volume (nameofvolume) appears to be OK 
If fsck found issues and has altered, repaired, or fixed anything, it will display this message:
*** FILE SYSTEM WAS MODIFIED *** 

Important: If this message appears, repeat the fsck command you typed in step 2 until fsck tells you that your volume appears to be OK (first-pass repairs may uncover additional issues, so this is a normal thing to do).
4. When fsck reports that your volume is OK, type reboot at the prompt and then press Return.
http://docs.info.apple.com/article.html?artnum=106214