Help!! iMac keeps shutting off

Every time I start it up it shuts off after a minute. All I get is a white screen with the grey apple and the spinning wheel and then it turns off. Before this I did the hardware test and I got this: 4HDD/11/40000004:SATA (0,0). I was also able to go on single user mode, but now I can't do either .

Anyone know what I could do? Thanks!

Yes I tried that, but the computer just shuts off

So when I turned on the mac today I hold the option key and managed to see the Mac OS X disc . So I just changed the startup disc to the OS X disc or should I change it to the Macintosh HD? I still cant login so my friend is going to lend me her laptop so I could do target mode though do I just drag the user folder to her desktop?

Thank you for your help!

My friend lend me her laptop but now the macintosh hd won't work it says it needs to be repaired I tried to with the cd but it won't work any help please?

What do you mean?...I'm trying to do target mode, but all that will show up in her macbook is the disc I have in my computer. It won't show the Macintosh HD .....Also now I can do single mode so I ran the fsck and i got this:

**/dev/rdisk0s2
**Root file system
**Checking Journaled HFS Plus Volume.
**Checking Catalog File.
disc0s2:0xe0030005(Undefined).
invalid mode structure
(4,1906)
**Rebuilding Catalog B-Tree
**The Volume Macintosh HD could not be repaired.
/dev/rdisk0s2(HFS)Exited with signal 8


should i keep doing the fsck?......Even if I try to reinstall mac os x it won't show the HD.