BSODs happen for a numerious reasons most of the time it's related to a hardware or a software conflict? Did they start happening after installing any new software or hardware. Also you might want to check the temperature of your CPU, the computer will often shut it self down as a protective measure once the CPU reaches a certain temperature, you can often check it in the CMOS settings, if you see the temp slowly rising even though the processor is not doing anything, you;ve got a problem. although you said you replaced the fan, did you replace the CPU fan? If so you might want to try replacing your CPU.