O.k. This is going to be a failed hard drive. What's happening is that one or more of the many sectors in the hard drive, that the operating system is written to, is corrupt or unreadable, so, the system reads to that point and past that point but can't put the two together, therefore it won't boot or run properly. It's kind of like reading a three hundred page book that has hundred or so pages ripped out of the middle, you need all of them to put the story together. Unfortunately, you will have to replace the hard drive.

Possibly, if it's working but running very slowly, it could still be a bad sector that it's having a hard time reading through, or it could also be a bad memory chip. Try removing the memory and starting with one chip in, then change them. If you see an improvement in the start up or it gets worse, then one of the chips has failed or is failing and will need replaced. If you find a failed chip, don't put it back n the machine. If the memory checks good, then it's still in the hard drive. It may run for a while, but eventually it will go out completely. I suggest backing up any data to disc that you want to save while it's up and running.