There'a a need to know more about your system. As so far understood, could be a driver problem, power supply, ram or HD failure. I would disconnect all peripherals except for C drive, and add back one by one. How old is your machine?
I have a Dimension 2300 that has stopped turning on, i dont even get a post or anything. I have a fx5500 on it and the monitor shows no signal transmitting.
when i turn it on the hdd spins and the p4 gets warm and everything is pretty much normal. the fx5500 fan spins, etc. the monitor will not show anything on the screen and the hdd stops reading.