Two things can do this to your PC: Bad Memory or Bad Driver. This isn't something easy for AE to trigger.
What version of AE?
what graphics card are you running?
What kind of computer?
I know you posted these questions on other forums as well, but these kinds of crashes really elude even the most telepathically gifted. I'm afraid this is really something that only someone can resolve who is actually able to see what beeps and messages your computer produces while it crashes. You can of course always do the usual of checking your graphics driver, other devices that may have bad drivers (see here for guidance) BIOS settings for your PCI bus and all that, but beyond that I don't see how we as forum users would be able to provide further help.
I know it is almost mission impossible to detect why my PC just restarts on this simple task: import DV, put DV on timeline, enter SPACE to preview DV -> PC restarts after 2-3 seconds. There is no beep, there is no error log, there is no error event in event viewer. Also I mentioned in my original post which configuration I have and that I tested memory already.
Next I will try to install AE CS5 on fresh Windows 7 64-bit OS without any drivers, after that I will try to change my ATI card to nVidia to see if it solves the problem. I will let you know...