From the start, I had weird BOOT behaviour - it BOOTS, shuts down, and re-BOOTS by itself.
So, today I did BIOS update (Latest for specific CPU and MOBO combo at ASUS - P8P67-M PRO BIOS "3602"!!!
Initially BIOS-update went well, but then Windows froze at start-up! Re-BOOTED, same thing.
Then, went back to BIOS, reconfigured CPU/RAM-FREQ relationship to (probably) a better state (not default).
Then again, when I re-BOOTED, Windows had a problem starting-up. But this time, instead of freezing, Windows recognised the start-up problem and offered "fixing" it. But again, Windows could not "fix" the problem. It re-BOOTED automatically, and now, Windowns seems fine. However, after this last reBOOT, some drivers automatically re-installed and I was promted to restart.
This happened minutes ago, touch-wood, no problems so far!
...Hope it may help anybody.
PS: I think the problem is with the BIOS/CPU/RAM relationship settings.