I agree with compdoc, try switching out the Memory. You've done about all the troubleshooting I could suggest but here's what's left:
Try a CD bootable OS, Linux live CD or miniPE BartPE (if you're comfortable with the questionable leval situation), ect,...
You could also try installing the OS on the drive in a different computer and then moving it to this one. It doesn't always work with XP, but if you manually install the drivers for the system it should boot properly. This would tell you if the problem is with the systems ability to boot, or the installation process.
OlyComputers
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