Sounds like bad
ram, I've had that happen to me before. Try this, remove everything except 1 stick of
RAM, the processor, and the video card, try booting it. If it successfully boots to an "Operating System not found" message or similar then turn it off and connect the main HD, boot up again, do this until you find the problem
If it doesn't boot up the first try then swap the
ram stick and try...it can be a lengthy process but it worked for me.
If the only thing you added was
ram though, that's what I'd bet it is
Also, can you please post the specs for this PC?