Well I installed the SM Bus Controller.
When you get fixated, you really get fixated....
The SM bus takes your temp....get past it.
Did you install full Mobo system drivers?
Could this be a video card problem?
Hardware-wise... no. If you installed the newest Nvidia drivers, uninstall them and install an older version you've used before and know works.
The only thing that would make a 'click' on a re-boot is a PSU....
I installed a new 400 watt PSU
What is it?
What USB devices are hooked up? You did this XP/drivers install backwards.