Windows 7: Overclocking Problem with AMD Phenom II x2 555

I have just built a new system consistsing of the following:

Asus M5A78L/USB3 AMD Socket AM3
Kingston 8GB (2x4GB) DDR3 1333MHz i5 Memory Kit Non-ECC CL9
AMD Phenom II X2 555 Black Edition 3.2GHz Socket AM3 6MB Cache
Western Digital WD20EARS 2TB Hard Drive SATAII 64MB Cache - OEM Caviar Green
Powercool 550W 80plus Certified Efficienty PSU - 1x PCI-E 4x SATA
Arctic Cooling Freezer 7 Pro rev 2 Socket 775, 1156, 1155, 1366, AM2, AM3 Heatpipe CPU Cooler
Windows 7 Home Premium 64-Bit SP1

The initial build of the system went as planned until I activated the CPU overclocker on the ASUS motherboard. When this is on the system seems to boot ok and starts to load windows but then hangs and all I get is a blank black screen. I would be happier and the problem easier to diagnose if it was a BSOD rahter than leaving me clueless. No other changes in the BIOS were made.

I have had to reboot and reset the motherboard to the default values to get it to work ok.

Has anyone had a similar issue or can help me identify what could be wrong?

You might not want to overclock right away if you've just built this computer. Give it a chance to "burn in" for a few days (better: weeks)...see if it's stable during normal operation to begin with. Then you can think about overclocking the proper way. I'm guessing that you used some kind of automatic overclocking and maybe it just pushed things too far this time.