I have a probleme With my Cpu . Wath is solution 

This is a proof of work question.

As I understand the algorithm, the past hash found is used to create the new hash,  but a nounce is added on the end.

Questions..

Is this nounce the thing that is changed to create a different hash?
The hashes are run until a hash with the correct amount of zeros is found, then the process starts again...

However, how does the hash get incremented? If you looked over the millions of hashes your GPU produces every second, are they all the same with a slight difference?

Thanks You 

I just got a new iMac at work (I'm a PC/linux guy, but my boss is a Mac guy). It has a ATI 6970, which is one of the best video cards for bitcoin mining today. So I figure I'll download Diablo miner and set it up to run with default settings. Apparently Open CL doesn't work very well on a Mac, so I was only mining at about 1/4 the rate that the 6970 would get in a PC, but still figured it would be worth it to run it over the weekend.

So I come back on Monday, and the computer is froze. I restart, and I'm getting visual artifacts on the screen, and it freezes, usually at the login screen, sometimes it logs in, only to have more video artifacts, and freezes shortly after. I'm pretty sure I fried the video card. I'm assuming that the iMac must have really bad cooling on the video card, and it overheated. Aren't there any safety features to shut down the computer if the hardware is getting dangerously hot?

I load in safe mode, and delete everything related to bitcoin or the diablo miner, and call the company's tech guy complaining my New iMac is acting up, acting like I have no idea why it would do that.