Why does my iMac freeze at startup?

***Not exactly sure of model as i cannot access it now... its 27" im fairly sure its early 08-09

The problem started when i was surfing the internet normally and then three fourths of the screen went black and began to flicker rapidly. The audio however kept playing. I decided to reboot via power button.

The imac would start up with the apple logo and the spinning startup wheel, but then vertical and sometimes diagonal lines would come up on the screen and startup would freeze.

I've tried powering up while holding the following buttons:

Option: My hard drive and a recovery hard drive load, clicking on either leads to a start up that freezes as described above

Shift: safe mode; the computer starts up as describes but a bar appears under the start up wheel, loads all the way then the computer freezes.

C: i inserted mountain lion with hopes of re-installing but holding c (and d for that matter) doesn't do anything different

command s: single user mode; i ran fsck it repaired the hard drive, ran it again said my hard drive appeared to be okay, typed reboot doesnt solve the problem

Rebooted manually ran fsck again typed exit, everything seems fine but freezes after " waiting for window server"

i reset the nvram, nothing different... im getting pretty desperate here any help would be appreciated.