I would recommend using a live CD of some type either Linux based or a windows based one so that you can determine if you're dealing with a hardware issue or a software issue. If you can successfully boot into a live CD's enviroment then I would say that you're working with a software error and that a repair or restore of the windows operating system might need to be done.
If you can not get into a running system off of a live CD then you might want to check out any new hardware that has been installed. Swapping out older hardware for newer installed hardware to see what works and what doesn't. Keep using the Live CD since you know that it's a working system and can't be changed. Remember to only change out one part at a time so that you know what you've worked with and what you haven't.
It sounds like you might have a fault memory module installed since you're dealing with an intermittent problem.
Good luck and the trouble shooting and hope this helps some.