Have you run through the steps in the My PC Won't Start sticky at the top of the forum? I do't think the heat sink would prevent the PC from booting unless it has already caused the processor to overheat and damaged it. Try stripping the system down to the essentials (Motherboard, processor, 1 stick of memory, display card, and power supply and then see if it will post. This will help eliminate other things like bad drives or other add-in cards. Also disconnect any external devices except the monitor and keyboard/mouse. Let me know how that works out.