|The fact that you had no problems running the x64 version of XP Pro for three months without problems right after removing the OEM copy of XP that came pre-loaded on the system indicates this might not be software oriented, or at least not likely OS or driver related.|
The chkdsk could be an indication of a failing HDD so I'd suggest running a manufacturer's diagnostic test on it ASAP. Also, the fact that it is shutting down arbitrarily could be related to overheating especially since you've had to have the fan, motherboard & RAM all replaced within six months of procuring the notebook.
Your HDD maybe the next on that list & who knows what component is going to follow after that. Seriously, should this be the case you probably need to ask for a new notebook because these symptomps depict a lemon buy.