HP pavillion desktop, hit the poewr button, lights up, fans start and immediately shuts off. Any ideas?

Disconnect everything from the motherboard except the ram and hard drive. If you have a known good hard drive, remove the one install and install the other one. Some power supplies need a hard drive to fully turn on. If the motherboard has on board graphics, enable the on board graphics in the BIOS. If it does not have on board graphics then you will have to use beep codes to diagnose it. The only thing you want plugged in is the ram, keyboard, known good non bootable hard drive, and power supply.

This link helps you understand beep codes.

http://pcsupport.about.com/od/nonworkingcomponent/...

Turn on the system and see what it does. Do you get a display from the on board display or a series of beeps. If you have on board graphics then you should get a message that you do not have a boot device if you install a different hard drive. If you do not have on board graphics then you should get a series of beeps telling you that you have no video. If you get nothing when you turn it on look at the diagnostic lights and see what they tell you. You have only three parts that could be causing the problem.

If you were using beep codes and it turns on then you can shut down add the video card. Turn it on and see what messages you get. At this point it should display no hard drive or no boot device.

If you were using onbroad graphics you should disable them, shut down and add the video card. At this point it should display no hard drive or no boot device.

At this point add the hard drive. If you have gotten this far then you can add the DVD and try to load Windows.

Usually that symptom is caused by a bad power supply.

Just put in a new power supply because it was doing this. same thing happens.

Could be some defective ram or the psu is defective