Try removing and reseating your memory modules one at a time with just your primary drive connected.  It is also worth removing any expansion cards you might have too.  If this gets you to the login screen, have a look through event viewer for any erroneous entries - you can then start shutting down, adding back hardware and starting up, one item at time until you find the offending piece of kit - assuming its hardware based.

Hanging boot and cannot boot to disk
In gas discharge lamps, for example, a current composed of ionized atoms excites the electrons of the atoms of the gas, and when they fall back to their ground state photons are emitted. Why is the reverse process (energy transfer of an excited electron to a free electron) never mentioned? Is it not possible or is the characteristic time involved too low for the electron to "wait" for an electron to pass? 

I have read about losing excitation energy as heat in chemistry oriented articles, but they don't go too deep. Here is an example.
