Just some suggestions:
New hardware could be causing "heat related" issues. (Motherboard chipset fans, heatsink, other fans, etc.) Have you tried removing the case cover, pointing a fan at the "inards" and running the computer for a while?
Is the power supply sufficient for the hardware you have installed. (400 watts, maybe more)
Depending on the amount of background programs you have running, they could be causing conflicts with each other. First, make sure that you have all the current audio, video, etc. drivers for the new operating system. Next, shut down all the unnecessary programs in "msconfig" and "services" by using the instructions and information at the links below:
Hope this helps.
I have installed a new MB and HDD. I installed windows xp home for my OS and all seems to be working fine but, out of the blue the system will reboot itself. It seems as if this mostly happens when I'm on the internet, yet it has happened while sitting idle at the desktop. any suggestions would be greatly appreciated.