Hello Runningman and welcome to PC Help.
There is a program called the BIOS that is built into the motherboard. When you first turn on your computer, it displays the black screen with white letters that shows system information and checks the devices on your computer. All of that precedes the loading of XP. The issue is not the hard drive, as the bios program begins and becomes visible on the screen before it even knows you have a hard drive.
The problem is either with your power supply or motherboard. Their is a slight chance the problem may be with the video card (if it's an add on), or the memory.
If you have a separate video card, then remove it and try the system with the video port that is on the motherboard. Although your motherboard may not have a video port. If you're comfortable with it and the motherboard has more than one stick of ram, try one at at a time.
I almost forgot the most simple thing to try. Unplug the computer, wait a half hour, then see if it powers up.