OK so when you push the power button it's like the computer isn't plugged in correct?
There is no test to use to see if it's your MOBO(motherboard). you can use logic and go through some trouple shooting and eliminate all other possible parts but thats about it.
What was the last thing you did before it "Died"?
CPU-Intel Pentium 4 630 3.0Ghz / 2MB Cache ~~OS Windows XP SP2
MOBO-Gigabyte 8I915P Duo Pro Intel Socket 775 (PCI-E)~~HDD-Western Digital 160GB(SATA)
PSU-Aspire 500W ATX~~RAM-1GB PC3200 DDR SDRAM (512x2 Dual Channel)
GPU-MSI GeForce 6600 GT(PCI Express)~~Monitor Viewsonic VA902B 19" 8ms~~ ATI TV Wonder Pro (TV Tuner)
~~Still to come~~: eVGA e-GeForce 7800 GT