If you've upgraded a lot of hardware, maybe you are overloading the power supply. I can't imagine that a squeaky clean system is suddenly going to get an overheating CPU, especially as you haven't upgraded the CPU, although, as Khana says, rendering is a fairly processor intensive job. Why not google for a temperature monitor (there are loads available for download)?
It would be helpful if you specified exactly what you've upgraded - it could be a video driver problem