Blue screens are never fun.
If it is reoccuring often, I would suggest the following:
Install Windows 7 (The least of many evils)
It is either:
Memory (RAM) corruption/damage
Data disk corruption/damage
In any case, back up your stuff (non executable, only media) and clear it all and start from scratch. If you still get a Bluescreen after a fresh OS install, then it is most probably hardware...
If you replaced your RAM and Harddrive and you still get it... it might even be bad cooling on your CPU.... strip it for the parts and get a new computer