1) IF ALL of the LED (Lights) were on they would use less than 1 Watt of power.
2) EACH fan uses 2 to 3 Watts of power.
3) A typical Processor can use 51 to 125 Watts of power.
Intel D945GCCR motherboard?
Can use up to an Intel Core2 Duo E4700, (800MHz FSB), according to Intel Support,http://processormatch.intel.com/CompDB/SearchResult.aspx?Boardname=d945gccr
An Intel Core2 Duo E4700 can use up to 65 Watts,http://en.wikipedia.org/wiki/List_of_Intel_Core_2_microprocessors#.22Allendale.22.2C_.22Conroe.22_.2865_nm.29
Check the Power Supply.
A Power Supply with a weak voltage power rail, will have plenty of power to light LED's, and maybe spin fans, but Not enough power to turn the Processor on.
No processor, no computer.
The computer isn't generating a video signal.
No Signal to the monitor.
Do you have access to a multimeter, or DC Volt meter?
Test the three Low DC voltages coming out of the Power Supply.
A) 3.3 Volt power rail
B) 5 Volt power rail
C) 12 Volt power rail (ALL are DC voltage)
[ In comparison two D cell flashlight batteries store 3 Volts DC ]
Need guidance with testing post back in a Comment.
Or if you have a KNOWN to be good, and Compatible power supply, that you can borrow for a test, try it.