IMac trouble login - need help badly

Question

I need real big help here. My wife iMac decided to act up this morning, out of no reason. She didn't download and anything from website or new apps. This is what happening now.
As usual, we type in the password and click login. It started as normal and show the desktop but almost immediately (2 seconds), it goes back to the same login password screen. We type the password again and again and it does the samething. We restart same thing. We shut down and let it sit and start again, same thing.
Now, we have so many important files and memories in this machine!!! Can anyone advise how we can get it started again normally without reinstallation?
MANY THANKS.

Answer 1

If you have the install disc, boot from it and use
Disk Utility
   1. Insert the Mac OS X Install disc that came with your computer, then restart the computer while holding the C key.
   2. When your computer finishes starting up from the disc, choose Disk Utility from the Installer menu. (In Mac OS X 10.4 or later, you must select your language first.)
      Important: Do not click Continue in the first screen of the Installer. If you do, you must restart from the disc again to access Disk Utility.
   3. Click the First Aid tab.
   4. Click the disclosure triangle to the left of the hard drive icon to display the names of your hard disk volumes and partitions.
   5. Select your Mac OS X volume.
   6. Click Repair. Disk Utility checks and repairs the disk.
Then reboot from your internal drive and repair permissions.