Boot trouble

So, my grandpa recently got an iMac to replace his old HP desktop, because the motherboard melted a little bit. The hard drive from the old was still good (or so we thought), so I tried to see if I could boot with it.

With the HP hard drive attached via USB, I tried to open the Boot menu by holding the Option key at startup. That didn't work (with our third-party keyboard OR the original Apple keyboard. So I did it through System Preferences -> Startup Disk, selected the HP drive and restarting.

It didn't work, something's probably wrong with the drive. Oh well. So I shut the computer down, unplugged the drive, turned it back on, and... it's still trying to boot from the HP drive that is no longer connected. On startup, it makes the standard startup sound and shows a blank grey screen. After a little while, the screen goes black and this text shows up:

"No bootable device -- insert boot disk and press any key
_

So, what do I do? How do I get it to boot from it's own hard drive again?
Just as before, holding Option through startup fails to bring me to the boot menu. I've also tried unplugging everything, including the power cable, for a little while and trying again, that didn't work either. Help!

 

Thanks for your responses.

I've tried many of those startup commands, but none of them seem to do anything. I've tried holding them at different times, mashing them continuously after pressing the power button, etc.

I'm starting to think the keyboard may not be communicating or something.. What is the green light normally supposed to do? Every time I press the power button, the green light comes on for a few seconds and goes off. It couldn't be the batteries, could it? The thing's barely been used..

I just tried it all again with the drive plugged in, no dice. :/

The iMac is quite recent, it's only been sitting in the house for about a week!

I was kinda surprised it saw the drive too; the Windows 7 startup animation even came on a couple of times. I didn't really think it would work just like that, just thought I'd see what would happen. >.< I didn't think it'd be so difficult to boot normally again!

 

@phoenixsan:

Read the post more carefully; I'm not trying to boot the windows drive anymore. I'm trying to get the iMac to boot OS X again. None of the startup commands are doing anything.