Suggest you have a bad power supply. Weak voltage power rail. Enough power to light lights, and spin fans, but not enough to turn the Processor on.
1) ALL of the light require less than 1 Watt of power.
2) EACH fan uses 2 to 3 Watts
3) A typical Processor uses anywhere from 51 to 125 Watts. Depends on what Processor it is.
If this is an HP Pavilion Media Center a1410n Desktop PC, it comes with an AMD Athlon 64 3800+ processor. (2.4GHz)
(The 64 stands for 64bit. It is a 64bit processor. The 3800+ is AMD's model number. The maximum frequency rate is 2.4GigaHertz)
http://h10025.www1.hp.com/ewfrf/wc/document?docname=c00607954&tmp_task=prodinfoCategory&lc=en&dlc=en&cc=us&lang=en&product=1841799The AMD Athlon 64 3800+ processor when operating at maximum capability, can use 89 Watts.
http://en.wikipedia.org/wiki/List_of_AMD_Athlon_64_microprocessorsWhat made the Power Supply fail?
1) Computer is dirty inside, as well as the power supply.
The cooling components for a power supply are Heatsink's inside, and the external fan.
When the cooling components get clogged up with gunk, the cooling capacity drops tremendously.
Heat = Wasted Energy
The power supply tries to keep up with the call for power, and eventually components inside the power supply fail.
2) Pre-built computer manufacturer's tend to put low quality power supply's in their computers. Saves them money.
Do you wish to know how to test your power supply? Do you have a multimeter, or a power supply tester?
One example of a power supply tester,
http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=5250576&CatId=1107Also, do you wish to know how to replace your power supply?