No reaction on power up

Yesterday I installed some new components in my pc.

I switched on the power, all worked fine until a restart. Then the pc is not responding. Mobo light is lit, also HDD led works, but no POST beep or booting sequence. Also the screen of my pc stays black.

I think my Power Supply is somewhat malfunctioning, but it easily could be the mobo.

Any comments on this? Would really appreciate it

Specs I installed:

- Intel C2D 6750
- Asus P5N-E SLI Mobo
- 500W Antec power supply
- 2 GB Kingston DDR-2 800 Mhz