(This is about my friends computer, which I can access, and I would appreciate any help about the below. In advance, if the provided data is lacking: my apology. Please let me know and how to do it (I am not a Mac user!). Thanks!)

System Specifications

• iMac Intel-based 21.5-inch model, late 2009.
  
• OS: Mac OS X Lion.
  
• 4 GB RAM(?) -- 2 x 2 GB RAM(?) sticks.
  
• Approximately 3-years-old.
  
• No warranty!

Problem

First, the computer was oddly slow for some time. The user decided to update system core software (system updates) as a hopeful fix. In installing a system update, the process froze, and I imagine the computer. In response, the user rebooted the computer.

On new boot, the computer could not actually boot: only the Apple logo on a white screen with an active progess bar is displayed. This remains seemingly indefinitely (the user left the computer doing this for a good few hours with no progress).

TL;DR The iMac will not boot and is stuck on the white page with Apple logo and circular progess bar.

Attempted Solutions

(The following is obtained from this support http://support.apple.com/kb/TS1587. The guide claims that if the following didn't work, then take it into their store. I just thought I'd put a bit more effort.)

• The used outlet is deemed to be good.
  
• The power cable was confirmed to be firmly connected.
  
• iMac was rebooted without any external hardware connected: nothing.
  
• Reset the iMac's System Management Controller (nothing):
  
  • Turned iMac off.
    
  • Unplugged power cable.
    
  • Reconnected power cable.
    
  • Booted iMac.
• Removed and reinstalled the SO-DIMM memory modules (the RAM?) (and cleared out the horrendous amount of dust): nothing.
  
• A variety of "safe boot" and other boots were attempted without success by the user (sorry for being vague).
  
• Booted while holding down the C-key with a Mac OS X Snow Leopard install disk already inserted:
  
  • Resulted in disk reading noises in combination with mentioned white screen for a good few minutes.
    
  • Then, the install screen and sequence slowly began.
    
  • In attempting to install this OS, in the dialogue for "Select the disk where you want to install MAC OS X," the process of "Examining available disks..." automatically begins.
    
  • This process with an active progress circle continues seemingly indefinitely.
    
  • In clicking anywhere on the mentioned screen or trying to use one of the available buttons the "spinning beachball" loading thing comes up and seemingly continues indefinitely (it seems to freeze here).
• Tried rebooting from end of attempt solution 7 and reattempting 7: same result seemingly (the user plans to leave the iMac on for awhile or all night to see if its does anything).

hum, is the harddrive full or something?

Have you verified the hard drive is any good? Could explain pretty much all the issues you are experiencing if it's dead/dying.

If there's any way to launch terminal during the install process, run the following command: diskutil list

Will running this command, if possible, give an explanation as to if the hard drive is any good?

If not, how can I check if the hard drive is any good?

During what we can get out of the install process, a menu bar does appear with menus such as "File." The only time I tried interacting with them was after it began "Examining available disks..." where I encountered the system (seemingly) freezing, twice. I'll try interacting with them before then

Thanks!