I bought an Athlon 1800 from ebay, and tried to install it in a Soyo mb. When I power on, all of fans work but no any response on screen. Then I tried the CPU on a working machine: XFX motherboard + Athlon 2600. When I installed 1800 on XFX mb, all of fans start but stop in 1 second. I suspected this CPU is defective. But worst of all happens, when I put the original 2600 back into XFX motherboard, it didn't work! Although all of fans ran, no beep, no screen. What happened? Was the power supply unit ruined? or CPU or motherboard burned? Could you please tell me what is most likely happened? Thanks!

Did you try CMOS clear?

Yes, I did! It doesn't work.