From what I've read, when a new block is discovered, the other machines will take a few milliseconds to verify if it's real or not first because say two people's computer discovered a solution within a second of each other. How do we know who the winner is? Well, again more luck. Other people's computers have to verify that your PC wasn't cheating/stupid/error/etc for the block. After a hundred or more confirmations, it's accepted by the "group" that your PC founding the winning block and everyone assigns the credit to your PC. So when two computers at the same time find the block, it's more luck on how fast it will be accepted by the group by how many other people's computers verify your find.

The chain is updated and if your neighbors computer was working on block 66777, it hears the news that your found the winning block and says "well, time to start on the next block" and begins the calculations over. Now your computer and your neighbors computer are working on block 66778, but each is doing it's own unique brute force. They aren't both working on the same brute force for the same block, otherwise, the fastest computer would always win over and over. The faster computer does have an advantage that it can brute force the next find faster than the slow guy, but the slow guy still has luck on his side.

Now imagine this with thousands of PCs out there playing the lotto every second. Just because you have a quad-core system churning 2,400 khash/s doesn't mean you are going to win every time. It makes your odds better of course, but by no means is it a sure thing just because of the raw CPU power you have. Ask the people at this forum who are running this on a server farm (like myself) and you'll see. I'm churning 10,000 khash/s on one of my systems (6 cpu) and it hasn't won a single block yet    I'm not bitter though, it proves to me that the system is being fair in the block generation, so I'm happy actually.