Does holding Option or alt key at bootup bring up a boot choices screen?
Can you boot from your Install Disc holding c key?
If you don't know the model, find the Serial# & use it on one of these sites, but don't post the Serial# here...
How to find the serial number of your Apple hardware product...
Might be time to replace the PRAM Battery, 4 years is close to their lifespan, far less if ever without AC power, & can cause strange startup problems...
See which one yours has...
The globe indicates the Mac is trying to boot from a network http://support.apple.com/kb/TA25439?viewlocale=en_US
You can remove the PRAM battery to reset PRAM.
Something to try.
Hold the 'x' key during startup. This will force the Mac to boot from OS X, no matter which disk is specified as the startup disk.
Hold the option key during startup. The OS X startup manager will appear, allowing you to select a disk to boot from.
1- mac operates only 2 keyboard comands. First of theese comands is option button. When i hold the option button, a pop-up which says "mac optional firmware password" appears on the screen. I don't remeber that i set any kind of password.
Secondly, when i hold (Command + Option + O + F) another thing appears on the screen you can see this image below
Mac demands another password on the screen
I also changed PRAM Battery and i dont think that there is a problem in HDD. Diffrent HDD tested by myself they didn't solve the problem. Finally,i tried to work my mac with SMU reset button but i couldn't make it work.
Please help me i have to make this mac works.
AHA, you have setup...
Firmware password protection in Mac OS X ...
It would block usage of all the startup keys, like C, N, T, D, CMD+s, CMD+Option+p+r, CMD +v, Option, and Shift, as well as booting from anything but the Hard Drive.
Force Removing Password Protection
1) Add or remove DIMMs to change the total amount of RAM in the computer.
2) Then, the PRAM must be reset 3 times. (Command + Option + P + R).