PC goes on black after using occt

Hello people,

I wanted to overclock my AMD Phenom 9850 Black edition. But before I started, I used OCCT to see if my non-OC system is stable by default.
I thought, well, in the bios is everyting on Default and Auto, so it SHOULD be stable.
Well forget it! After 5 to 10 seconds using OCCT my pc went black (well, the monitor) and there followed immediately a reboot.
This isn’t good I think. What can it be?

Specs:
Motherboard: gigabyte ga-ma790fx-ds5
Processor: AMD Phenom I 9850 Black Edition
Graphics card: XFX GTS 250
RAM: 4 GB 800mhz DDR2 from Corsiar

I hope this is enough information. If not, please tell me so!

(If my english isn’t good, sorry, I’m a 14 year old dutch boy ;))

you might be pushing it to hard or your cooling is insufficient. proceed with caution.