A bad CPU, memory stick, chipset, motherboard? run it with one stick of memory at a time for starters. But it can be a time consuming process to narrow down a bad piece of hardware.
What parts did you replace?
I have replaced the power supply(new) CPU fan and shroud, and installed 2G memory upgrade. I also formatted the hard drive and installed Windows XP with service pack 3.
This unit did not have a power supply, any memory or fan and shroud when i aquired it.