I have a Dell Dimension 2400 running Windows XP Pro SP 1a. It's all Dell standard stuff. No extras. In the middle of working (doing anything) it will instantly shut down as if hibernating and the HDD LED on the front panel blinks amber. After that I cant power up or down by pressing the power button once. I can only press and hold to the power button to force-shutdown completely so that I can power back up. The cycle repeats itself whenever it feels like it when ever it wants.