I am no expert, but I certainly would not start my computer without the CPU heatsink and fan properly installed, patience is a virtue as they say. Would you rather risk frying your components to make sure you didn't shock them to death? Also, from what I read, you will have to remove and reapply thermal paste anytime you remove the heatsink, and without the heat sink being screwed onto the processor, the thermal paste is not likely to properly spread anyways (which could lead to overheating). I am assuming you bought an aftermarket CPU cooler, did the stock cooler come with the appropriate mounting hardware? You could fully attach that one and I believe it should be good enough to POST and even use the PC while not under heavy load (Although someone may correct my assumptions).