The problem with fsck and Disk Utility is that neither check for bad blocks on a drive. They correct index problems. If a file has become corrupt and it can't be indexed then and only then will they identify it as a problem, otherwise the problem persists.
I'm guessing the hard drive is bad. I'd try the following:
- If you have the install media boot from it just to make sure the system will boot completely at all. If it will it's good because you know the system is working. If it won't then you've got system problems.
- If you can gain acccess to another bootable external hard drive, try to boot from that. If it comes up, once again it implies the system is OK.
The odds are the HD has simply gone bad. The symptoms you're describing are those of a drive failure. We use Scannerz to test our stuff and the company that makes the product has lots of info and downloads about drive problems that may be of use to you. Here's a link:
I wouldn't rush out and buy their drive testing product without doing the other things I've mentioned first because if your system is at fault you would just be wasting your money.