Wow, someone else did the exact same thing I did 2 years ago. I bought a Dell D620 in the spring of 2010 from eBay. A second hand unit. The person who I bought it from described a problem where they would be able to load into Windows for a few seconds before the screen goes black. Since I'm a PC junkie, I figure that I could fix this problem. I took the challenge and I am "somewhat successful" in fixing my unit.
Basically, the computer will overheat, then the screen goes black and the power light will still be on. You will have to hold the power button down for four seconds to shut the notebook off completely.
Problem: Something is defective or broken on my Dell D620 unit that prevents the fan from automatically spinning up.
Things I had done with my unit:
Dust bunnies in heatsink vent area. Yep. Clean with compressed air can and vacuum cleaner.
Replaced "supposedly" broken cooling fan with a new OEM unit. Yep.
Replaced the heatsink from a single prong unit to a dual prong unit. Yep.
Used Arctic Matrix Thermal grease on CPU and heatsink. Yep.
I pried off the small panel that covers the power button and removed 3 screws holding the keyboard down then I flipped over the keyboard and used a tabletop fan to cool the CPU to get to the next stage:
Updated the BIOS to A10 in Windows.
Disabled Intel's Speedstep in the BIOS. Effectively limiting the CPU speed to 1.0Ghz. I did this to reduce heat generated from the CPU. If I didn't do this step the notebook will still overheat.
Downloaded i8kfanGUI from http://www.diefer.de/i8kfan/ and installed the program in Windows.
In i8kfanGUI, click on "Force Fan Speed to High" and there is also an option somewhere in the program to make sure the program loads when Windows starts up.
Using i8kfanGUI, make sure the core temperatures of the CPU never reaches 55 degrees Celcius, the point which the system locks up and the video goes black.
Shutdown the notebook and place the keyboard and power strip panel in the right positions.
Hopefully this will fix your problem as it fixed mine. :)
By doing this, my notebook became an "Undead" Notebook. If you take it outside during hot weather, it will still overheat with the same symptoms. If you use it in an air conditioned room it will stay on.
The downside to this fix is that the battery drains faster because the fan is always on. To combat this, I have purchased a secondary modular battery that goes in the DVD drive slot. I get around a total of 4 hours of total battery life with the two batteries in the notebook.
Intel Core 2 Duo 2.0Ghz, running at 1.0Ghz with Intel's Speedstep disabled.
120GB Western Digital Sata Hard Drive
Onboard Intel Graphics Chipset
Windows Vista Business OEM 32-Bit. (Previously Windows XP Pro)