Here the results on my first attempt and my first post on this forum as well...

I succeeded unlocking my Phenom II X2 550 on an AsRock A780GXH/128M (BIOS P1.20) after having installed Win XP SP3 (see CPU-Z link and screendump below). I used default BIOS settings, put Advanced clock calibration (ACC) on 'auto' and Cool 'n' Quiet to 'disabled'.

Prime95 blend ran for 15 hours stable, but in return I have boot problems now!

After 'hibernation' or 'restart' the system doesn't even boot into the BIOS anymore (no startup splash screen, just black screen, no mouse lighting, fans are running). Only after many reset button / 5 sec power button pushing, it seems to revive and (if hibernated) windows says it failed in resuming from hibernation, then starts fresh windows and runs ok again (still showing 4 cores in CPU-Z).

The BIOS max Vcore setting is at 1.55 V (default value) and the cores run at this max voltage as well (see screendump).

Later I tried to step back to BIOS P1.10 and unlocked it with ACC -> 'auto'. Although the default max Vcore value is the same, the funny thing is tha the cores are now running on 1.40 V and the system is not stable at all, crashes in seconds / minutes. The CPU model is not recognized by this older BIOS btw.

Any suggestions to fix my boot problem are very welcome!

CPU-Z dump: http://valid.canardpc.com/show_oc.php?id=603524