Ok, so I built a computer (not my first and DEFINITELY not my last) and when I plug it all in and push the power button, the fan comes on for a split second and then shuts off and nothing else happens. Is this a power supply issue or a MOTHERBOARD/processor issue or what??? It is an Antec case w/380W power supply, EliteGroup MB P4M800PRO-M (P4/1066), Intel P4 511 Box 775, and a Seagate 120g Harddrive. I also have ANOTHER fan besides the case fan and a cd rw/dvd rom drive. Any ideas?With a new build it is important to benchtest the system BEFORE it is put in the case. It is also important not to put ALL the pieces together and then be left wondering when something doesn't work what the culprit is.

Disconnect everything except the CPU/processor, 1 stick RAM, the video card (if separate) and the keyboard/mouse. Try to boot. What happens now?

If this works OK, you can add one thing at a time and TEST to see what is causing the problem.

Bad CONNECTIONS, motherboard shorting on case or CPU heatsink improperly applied are the first things that come to mind here.Did you remember to use the thermal pad/paste on the CPU ? ?

Is this a used PSU ? ?