Well, this shouldn't be too hard to solve (or at least figure out what the problem is), provided that you have another computer at your disposal. First, you should know how to swap a hard drive, because this is what you'll have to do.
1. Stick the hard drive in another machine, and see if you can boot the machine with the old hard drive (the one that's in your current computer right now).
2. If it boots perfectly, and with no error messages whatsoever, this means that you most likely have a serious hardware problem other than the hard drive (such as the motherboard).
3. If it still boots with errors, either the hard drive itself is shot or the data on it is corrupted. Basically, put 2 hard drives in the secondary computer, boot off the original hard drive, and back up any important data. Next, run a Scandisk on the hard drive that was in your current computer, and see if it comes up with any hardware or software related problems.
If the hard disk is alright, but the data is corrupted, you will need to reformat your hard drive. If the hard drive itself is bad, you will need to buy a new one. 100GB hard drives are relatively cheap, so don't get a lousy one (Seagate is a good company).
Hope this helps
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Vampirical Lurker
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since Apr 2006
Unsolved 
But no hard drive works on the computer 
Theres just a blank black screen on the screen but thats just means that the moniter isn't recieving signal. I think that both the motherboard and the hard drive is broken but my keyboard caps, numlock, and mem. light flashes when I turn on the power button on my power supply or my electric switch to my room that gives power to my computer. Also when I plug in the ethernet cable to the computer the light where it's connected to. 
