I don't know if you guys have experienced this. I installed W2K and 5.13 and ran for a
week with no problem. I put in Coolbits last weekend and tweaked my Core and Memory to 150/180 since I am using Diamond V770 only. After playing my game for about 2 hours my PC freezed. When I reset and reboot my PC I got BSOD and complaining about error in NTOSKRNL.EXE. I tried SAFE mode, Setup Recovery and last good config etc. with no avail. Finally, I gave up and installed W2K
again from FDISK, format my HDD using NTFS and setup. My PC is now up and running, but
what could be the cause of it? Any suggestions?

somthing dammaged the file on your computer when you restarted i guess. i had something similar to that happen, where that file got currupt...

I'd say your problem was probably related to the Nvidia drivers. Did you try reverting to win2k drivers or older Nvidias before reinstalling? Think that would've fixed the problem. I had a similar problem also driver related. Btw, MS knowledgebase on their website contains numerous incidents of this kind, often with a solution aswell. Try there next time and maybe you wont have to reinstall. Regards.