There's so many things that could be going wrong.
I'd say first disconnect all external devices (USB Ps2 etc) including mice and keyboards and see if that fixes the problem
if not, try connecting via safe mode (press and hold f8 key)
If this works you know your computer isn't falty (if it doesn't you should try booting with a single stick of ram and changing it out etc)
does your computer shut down after it goes blank?
If you can get into safe mode run some antiviruses (Spy bot S&D, Microsoft essentials, avast, malewarebytes) etc. [i'd personally reccomend malewarebytes]
let me know if you have any questions/can clarify somemore.