Hello, I just built my new computer but when i hit the power button, the power light flashes for 1 second and my processor fan and case fan spins for about 2 seconds but the computer will not start.

AMD XP 64 3700+
GEFORCE 7800 GT
WD CAVIAR 80GB HD
CORSAIR 1GB MEMORY 184 PIN (2X 512MB)
ASUS A8N5X
400W POWER SUPPLY + CODEGEN CASE

Please help!