how to tell what's broken?

[Guest]

lets say I order some parts to assemble a
new pc.. and it refuses to boot when put together.
how can I tell if the memory, motherboard, graphicscard
or powersupply is broken? and wich one it is

 

[RavenPrime]

Step by step.
Power LED lights up, HD spins up-->Powersupply probably OK.
BIOS setting menu accessible-->Motherboard probably OK.
Video appears on monitor-->Graphics card probably OK.
On bootup, the system does a memory self-test--there is a BIOS setting to do a complete (rarely necessary) or quick memory test.

James