Won’t pass POST test

I am working on a HP Pavillion xt936 (1Ghz 128MB Ram 40 GB HD with a AMD Athlon CPU) and it won’t boot up and won’t pass the POST stage. I never get anything on the screen. I have replaced the video card (PCI) RAM (PC100), PWR Supply. I have disconnected all the cables except for power to the (Trigem) motherboard and get the same results. Today I replaced the CPU, (being told this is where the problem is) and it is still not even coming up to the POST stage. Has anyone got any ideas? Please advise ASAP….Thx