PC Just Stops

I have an endusers PC that I have replaced the motherboard in. The PC worked just fine last week.
Today the PC will boot up and then once the enduser starts to work, she looses her mouse and keyboard.

I have replaced the video card and it is still freezing when she starts to work. We have to power off the machine to get it to reboot.

What else can I check to see why it is doing this?